First, measure how big you want your display(s) to be. Next, cut out your foam-core and cardboard to size, and glue together. Then, iron and cut your fabric of choice at least 8" wider and longer than the board, and place the board with the foam-core side facing the fabric, with four inches of fabric surrounding each side of the board.
Now, take your staple gun. Fold the fabric over one of the long sides , pull snug, and staple four times along this side. Do the same for the other long side, but pull the fabric taut, being sure the fabric is tight around the board, but the board is not bending. Staple twice at each of the short sides, but to not get too close to the corners.