Having selected a prototype (see Design in Detail: Prototype Selection) its time to get alpha playtesting. For me this consists of two stages, closed alpha and open alpha. Closed alpha is where I sit around playing my game against myself, usually entirely in my head, writing and re-writing it until its very basic mechanics work. This stage has two purposes. Firstly, while the open alpha will only involve close, trusted friends that's still no reason to waste their valuable time. So you should use closed alpha testing to ensure that there's something worth presenting to them. Until you design and produce your prototype and push the cardboard around the table you'll probably find a few parts of your game to be practically unplayable so this is your chance to find and fix them. The second purpose is to work out how to be your own designer and developer. Many games give these jobs to two different people and the relationship between them varies depending on the individuals involved, but in broad strokes the designer tends to originate the idea of the game and the developer engineers specific elements to work smoothly. Playing both parts yourself requires some clarity of purpose at this stage. As designer you have to have a clear understanding of your game's spirit. As developer you have to get the thing running, spirit be damned. I'm lucky to have been on the developer end of a very rewarding designer/developer relationship. As developer of Gaslands, I had to be free to come up with whatever worked for the current mechanical problem we were facing, even if it stomped all over the spirit of the game. It was the job of the designer (Mike Hutchinson) to defend that spirit against my spanner wielding assaults. With SSO I played both roles, which was quite tough since as developer its very easy to end up with tunnel vision and wring all the fun out of your project. Even if you don't, when you open yourself to playtesters they will with all the love in the world try to make your game into their game; you need to be ready to defend it from them and to do that you need to know which parts of the game are vital to defend and which you can let go of.
Once you've got your game into some kind of decent shape and have a good idea of what makes it worth fighting for you're ready to open your alpha test. You need to alpha test with smart, insightful, patient, patient, people that you really trust to be open and honest with you. If you know them ahead of time you might be able to get a blind playtest before getting too far along in the process. Blind playtests become very difficult to dig out of people so do take the chance to claim them while you still can. Your alpha testers will be playing your game tens or hundreds of times so they're likely to be friends or colleagues but you need to trust them not to protect your design or feelings because they're your friends, which can take a lot of honesty and understanding. You'll also need them because they will see the development of the game, beta testers will tell you if and where your game is fun and where its confusing, but they will tend to offer solutions that you and your alpha testers have already attempted.
Try to talk your alpha testers through your process, talk to them when issues come up and try to keep notes along side your testing as much as possible. You'll probably feel the need to keep your thoughts and solutions from a playtest private, and when you're in open beta that's probably a good idea, but try to let your alpha testers in more. The purpose of alpha testing is to make the game work, which means a lot of playing it when it doesn't. Polishing, tweaking and adjusting until you find awesome comes later.