Step 1: Acquire Corks
For a drinks coaster sized mat you'll need 8 corks, for a 19cm x 19cm mat like the one I made you'll need 64 corks.
Step 2: Base and Frame
You'll need to lay out all your corks, measure the overall size and work out how big you need the wood.
A nice wooden frame around the corks will protect the corks from getting bashed off and looks good too, take this in to account when making your base board.
I recommend thin plywood. I also think having adhesive pads on the bottom are a great idea.
Step 3: Glue on Corks
Hold down each cork for 5 or so seconds to ensure a tight hold.