What designers should do in relation to protecting their designs when showing them to others is, for obvious reasons, a concern for many starting designers. The short answer is, don't worry, you don't actually need to do anything. There is a good deal of professional advice freely available on this subject to anyone with access to a library and I would strongly suggest referring there if you have any worries or doubts. My opinions are formed from a range of sources but two excellent places to start are the "For Dummies" range of books for general legal advice and Jeremy Holcomb's "White Box Essays" for an industry specific view point.
I'm not going to try to give advice for all people, because I'm not an IP (Intellectual Property) lawyer, my advice is intended to be applicable primarily to being a small or first time independent games designer. Quite simply different things are practically possible or realistic for a major company selling tens or hundreds of thousands of units than for a first time designer or self-publisher who will be lucky to sell their first thousand.
There are two reasons you don't need to do much, first of all, copyright is automatic once text is in a fixed form and secondly people don't want to rip you off. They don't want to rip you off because;
1) Ideas cost very little relative to the final price of a game. Development, artwork, manufacturing and distribution are expensive and publishers will not risk losing out on their investment in those over the cost of owning the idea they are based on.
2) Its a small industry and nobody wants a bad reputation within it.
3) Designers mostly want to work on their own ideas, which they don't have time to develop, without stealing someone else's.
4) Designers give a huge pay back in terms of enthusiasm and work for very little outlay. A first time designer will practically work for free and publishers know that.
Clearly cases of IP infringement have occurred in the industry but they are both massively in the minority and not relevant at the scale a first time or self-publishing designer is working at. Instances of IP infringement tend to fall into one of three categories:
1) Open counterfeit, which you have to be publishing tens of thousands of units to attract.
2) Non-branded versions, such as those copying Guess Who or Operation etc. Again, you should be so lucky to sell units sufficient to attract this sort of attention. Maybe if you have the next Story Dice, but its not likely.
3) Using IP which attracts other designers. A lot of IP issues for independent designers come when they take a generic approach to a very specific IP without securing the rights to it. If I imagine my first version of a Blade Runner board game it will be very similar to yours, so if I make a spec version for Blade Runner I risk overlapping your version. Either make your game unique, secure any specific IP or avoid using someone else's IP in the first place.
In general terms there are three forms of IP protection, copyright, trademark and patent, potentially increasing in cost and level of protection. Additionally there is the non-disclosure agreement sometimes used in personal interactions. They also increase along that list in how bad of an idea they are. If you're willing to take my word for it just use copyright and ignore the rest, but feel free to read on and find out my reasoning for each.
You don't actually need to do anything to obtain copyright over material you have created, simply creating it grants you copyright protection. You do not need to put the copyright symbol on it or put on your name and the date (©Joe Blogs 2019), however you should, not as a requirement but rather to indicate your intention to protect your copyright to others. Particularly with the growing popularity of ideas such as creative commons licence, marking your copyright intention helps to clarify against accidental breach. Copyright protects you against a direct replication of your work but not its processes. For example, if you write a recipe for toast you can copyright the layout of the text, specific order and wording of phrases or photos, however another writer is free to write a recipe using exactly the same ingredients or processes. In short copyright protects you against people actually copying your game but not against them copying a mechanic or other element. It also stops them stealing your images, phrases and names. This should be more than sufficient, in the event you think that you should protect a mechanic or process more specifically please consider that game design is a collaborative community and that your mechanics will contribute to the growth of games design. If that isn't good enough consider that a game referencing your mechanics will lead to increased sales of your game.
You can register your copyright if you have a final copy of your game (because you'll have to send a copy into the office of registration and it will need to be in its final and fixed form). It is not free but if you're self-publishing its the most you should really spend up front on protecting your idea.
Again you can trademark things for free, in fact your things are technically potential trademarks automatically. Trademarks have to be both distinctive enough to not be confused coincidentally with a similar element (for example Harry Potter cannot be made into a trademark unless its written in a distinctive font, while Lord Voldemort can) and have to be on sale before they can be protected. By placing the TM symbol next to an element you signal that you intend to fight for your trademark if you consider it to have been breached, nothing else. As with copyright you can register a trademark, again for a price. The problem with registering trademarks is that unlike copyright each instance of a trademarkable element needs to be registered separately and even a single small game will contain enough trademarks that registering them all will probably cost more than a small independent game is likely to make.
Patents are complex and expensive (by expensive I'm talking tens of thousands). They take a while to register and although in theory anything can be patented in practice most games will have very little that a patent would be applicable for. That aside there is a more significant reason that patents are a terrible idea for an independent games designer. The patent process requires you to protect the details of your idea until the process is largely complete. If I'm making a toaster I can tell if its faster or more energy efficient without ever showing it to another human being, meaning that I can tell that its worth patenting all on my own. Nobody can tell if their game idea is any good without showing it to somebody and they certainly can't develop it fully alone. As an independent designer, developing and patenting your game at the same time is practically impossible, and developing should win out over patenting. There are patented games out there, they universally belong to companies with the resources to develop a project using multiple people under contracts over time and with distribution deals in place to guarantee a return on their investment. Additionally such patents generally refer to a physical element of a game such as the "pop-o-matic" or the "hero clix" bases. Its possible that you're reading this having a homemade physical element in your game similar to those elements but if you are considering patenting a game mechanic its probably not a realistic idea.
In short, don't touch these with a very long barge pole. I include them because they are technically a way of protecting your IP but there is really no reason to use them. If you are considering them you are presumably doing so in relation to playtesters or publishers. The idea of asking a playtester to sign a NDA I frankly find rude unless you are at least paying them and if you can afford to hire your playtesters you should be giving them proper contracts based on better legal advice than mine. If you're not paying them then they are doing you a huge personal favour that your game cannot happen without and you want them to sign a contract which will allow you to sue them because you think they are going to rip you off. I really shouldn't have to explain how distasteful that idea is.
Some designers try to get publishers to sign NDAs prior to pitching to them. Considering how hard it is to even get a pitch meeting with most publishers I do find the nerve to attempt this impressive, but it is a terrible idea. Mostly because it suggests that you will be a difficult person to work with but also because even the smallest publisher will have several games in development. Even most independent designers have four or five games bubbling away. The odds are that your game will overlap to some degree with one game the publisher is developing either in theme, mechanics or genre, so asking them to sign a contract allowing you to sue them if something does overlap is not in their best interests even if they have no sinister intentions.
In conclusion, in games design until you show your game to someone it doesn't exist. Show it to as many people as you can as often as you can, and don't let lack of IP protection stop you. In the end massively more games have failed due to lack of development than have ever come close to suffering some form of IP conflict.