You will need:
#a number of stamps
#thick piece of cardboard
1. My piece of cardboard is a bit smaller than A4 so I needed quite and few stamps. They are getting harder to collect now since the art of letter writing is dying.
2. Glue the stamps down using pva glue. I overlapped mine so there would be no gaps. When dry cover the whole thing in a watered down coat of pva glue. Wait til it's bone dry again ( the next day) and apply another coat. You could always cover it in clear contact too if you wished.
Mine was a work in progress for a couple of months while I souced more stamps to cover the board. I'm also cresting one with Christmas stamps. This one will take a while. I've only got a couple of rows so far.