I've seen lots of bike seat locks made from bike chains that look stupid - they're giant loops of bike chain haphazardly looped around the seat rail and the bike frame that flap in the wind. Effective, sure, but they look like crap. Who wants that?
My bike seat [bike] chain lock really isn't anything wildly different, but I think it looks quite a bit better by using a little less chain and shrouding the chain in a piece of innertube.
Step 1: Materials, Tools
Clearly the most critical part of the equation is a bike chain:
Go to your local bike shop and nicely ask for an old chain that they've discarded. The nice bike mechanic should oblige you with some old chain - 2 feet should be more than enough. If they say you'll have to buy one, I'd go to another shop - bike shops usually have these things in the garbage can so I wouldn't pay for one (although bike mechanics always like tips - throw em a few bucks and the next time you get your bike serviced there, there's a good chance you'll get A+ service).
Greasy chains are good. Although they're messier to install, the grease ensures that the chain won't rust and deposit rust marks where the chain is looped around your frame.
You'll also need a chain break and around a foot of innertube. I use some 700c by 18-23. Certainly you can use any size that you want, but like I said earlier: loose = stupid looking; you don't want anything flapping in the wind.