I use watercolor and printmaking papers available in art stores. Some of my favorites are:
Copperplate paper (used for printmaking)
Rives BFK (also a printmaking paper, gives slightly chalky images)
Canson Student Foundation Watercolor (the cheap kind)
American Masters Watercolor
and whatever else is on sale or strikes my fancy. More expensive papers do not make better prints, as far as I can tell.
I typically print on pieces that are 1/16th of a sheet of 22 x 30 paper. Approximately 5 x 7".
Step 1: tear or cut the papers to size
Step 2: mordant the paper - papers are made from vegetable fibers, usually cotton and sometimes unspecified cellulose which can be treated the same way. A mordant creates a permanent bond between the natural color and the fibers to increase the light-fastness and range of colors. I use alum (an aluminum salt used in pickling) in the form of alum acetate, a little iron (rusty nails in 1 part vinegar - 1 part water) and a little copper (corroded copper pipes in the previous type of vinegar solution or a tiny dash of Miracle Grow - which contains LOTS of copper) The solution should be colorless or slightly orangey-brown. I use left-over fabric mordant at 15% alum, less than 2% iron and less than 1% copper by weight of fabric. The weight ratio is not the same for the paper, just a guide for making a solution. I usually soak my papers for 4-24hours but you can also paint a mordant solution the surface of your papers with good results. I enter the papers to the solution one at a time to make sure they are fully exposed. Paper may be dried after mordanting and re-wet with tap water before printing.
Step 3: Lay out plant material on damp, but not dripping paper. Paper, plants, paper. Paper, plants, paper. This reduces some of the secondary prints running through and I never have to decide which of two printed sides I prefer. If I want exceptionally clear prints with no 'background noise' I put a piece of baking parchment paper between the 'sandwiches.' (Paper, plants, paper, parchment, paper, plants, paper, parchment.
Step 4: I have experimented with a number of methods of steaming, but my current favorite is clamping the pile of papers and plants between two porcelain tiles ($0.99 at Home Depot) and securing them with 2" spring clamps ($1.88 ea). C-clamps work as well, just tighten slowly and each corner evenly or the tile will break. I am not trying to crush the leaves, just to have an even contact between the leaves and the papers.
Step 5: Steam the whole press. I use a large turkey roasting pan with a canning jar lifter in the bottom. I place the clamped tiles on the jar lifter above boiling water, replace the cover and steam for approximately 1 hour. I un-layer the papers as soon as I can touch the tiles and clamps. Some practitioners let their pieces sit overnight, but I've found that I get brighter colors if I open them sooner rather that later. Sometimes the leaves stick to the paper. Just wait until the paper dries and they'll peel off.
Remember, you're only committed to several sheets of watercolor paper, experiment, play!