I figured I would share my technique as it is featuring heavily in my current build. You can take, leave or add steps as you see fit.
Apologies for my sub par photos with dodgy lighting.
Start with some card. I picked up mine from WH Smiths. It should be a bit thicker than normal printer paper.
This is stained using two regular tea bags to give a patchy base color. It is dried before it is worked on further.
It is then cut into strips and small rectangles.
Card by itself is too thick when looking at it in scale so use some 100 grit sand paper to sand the edges of each sheet down. The sanded side becomes the back of the parchment.
If you want distressed parchments then use 60 grit as it will naturally remove bits and make holes.
When you flip them over they are ready to be worked.
You can then add a splash of color using colored pencils.
Then add small lines of text using a mechanical pencil.
Once that is done do a thin wash of brown streaked onto the page from the edge to simulate wear.
Then a very thin wash of green in the extreme edge to simulate mildew and heavy wear.
Here are two sheets sat on a sheet of the original card to show the difference in color.
Last step is to use a mandrill to round the edges a little bit. Be sure to ink the reverse side if the edges are going to be visible.
From cutting the strip to finishing two pages only took about five minutes. I have been doing about ten at a time to keep my sanity. The process is pretty quick if you are not taking photos step by step. I am building up a store of them as they feature heavily in my current build.