I've done a little screenprinting at home and it is a ton of fun. Here's a few tips to help you along in handy list form (I like lists).
1. To start with, I wouldn't try to make screens myself... to get good results it has to be tensioned just right, the frame has to lay perfectly flat, etc. Honestly, I'd splurge on the screens and just save the hassle. They aren't too expensive once you figure in that with care they will be reusable for a long time.
2. I made a press loosely following these plans http://www.screen-printing-t-shirts....ess-plans.html
. I made a few changes since I wasn't doing t-shirts but it is a good start.
3. I'm not sure how well the materials you want to print onto will take the ink... you may want to buy a small jar of ink and smear a light coat onto the plexi and see if it will dry and not flake off.
Anyway, hope some of this helps and have fun with it.