pretty involved, really, for something perceivably so simple. Unless you're wanting to build a DIY wavemaker, I'd say go buy a simple appliance timer. Can't cost you more than $15 each for one of them fancy new digital ones.
There are a few DIY plan that based on the famous IC555 chip, and other similar IC, but it is not worth the time and money unless you know what it involved. It is a good learning experience, however.