a basic flower press works well...acid free rag paper on either side with corrugated cardboard. instead of a press a good stack of books ...the rag paper will absorb minor bits of moisture and "cushion" some of the more delicate aspects.
This is how we prepared herbarium specimens when I was working on my masters in botany classes
I've heard that Glycerin works well for preserving leaves, although I haven't tried it myself. I think you can buy it at Drug Stores.
Press between two pieces of wax paper with an iron set on low. Used to do it that way for science projects as a kid