Step 1 Cut a rectangle piece of 1/4" foam core board. You can get a full sheet which is almost 2' x 3' for about $2-3 at any of the major craft stores. Cut the piece with an exacto blade or hobby knife that is sharp (new blade) so you do not tear the foam which will happen with a dull blade. The size should be the size you want for the models you are using. In War of the Ring case, you use the measurements provided in the rule books. Cavalry trays, Infantry Trays, Tray for the 9 wraiths as a unit, and a tray for siege bows. So you really can make all the trays you need this way and if they do get damaged (highly unlikely) then they are cheap to replace.
Step 2Trace your base size as many times as needed in the locations you want to place the models on the tray. In this case I am making a infantry tray with a 1" base. I will place 8 of these circles on the base as 2 rows of 4.
Step 3 Cut through the top card layer. !!!ONLY!!! cut through the first card layer on the initial cut, this will make it easy to only cut the foam in the next step so you don't cut through. It is not the end of the world if you do cut throughout too.
Step 4 Now you should be able to poke in through the foam and feel the resistance when you hit the card on bottom. Reach that trace that circle again with the knife only cutting the foam layer.
Step 5 Poke your finger in there, don't be afraid you won't hurt it. And dig out that little circle. You are separating the foam from the card on the other side. Some Foam board is glued better than others, trust me you will know when you do an easy one then a hard one, but neither is difficult, just one is cake.
Step 6 You can start basing it right here if you want. It can be used just like this also. If you want to give it a little more strength and support to last many battles then you can wrap the edges and bottom in card stock. Card stock can be a back of a notebook, or a cereal box or other packaging in card.
Step 7 Base it with PVA white glue (or elmers Glue all is cheap and works fine) and sand and flock. Paint if to suit. You can make a lot of these with a sheet of foam core.
Here are examples;