Never built a greenhouse, but I have some Ortho gardening books and they never done me wrong. Here is a greenhouse book from those folks:
Earl, you'll find tips, as well as plans, from the University of California Extension Service here:
(The title "Small Plastic Greenhouses" on the cover is something of a misnomer. The article is actually quite comprehensive.)
Thanks for the answers, but I'm looking for something that is VERY small, actually portable.
Sounds like what you need is maybe a cold frame, not an actual greenhouse. Here is a good tutorial on making one of those - http://www.popularmechanics.com/home/improvement/outdoor-buildings/4345342
Definitely portable. Also, some people use discarded windows or glass doors for the lids of these frames. Might be worth considering as a cheap, eco-friendly alternative.
Thank you Steve G! I guess at my age I still don't know how to ask the right questions!!!!
Glad I could help. Let us know how it works for you.