Laminate floorboard scraps
Quarter round scraps
Some sort of clamp
Step 1: Gather your scrap
Step 2: Cutting the frame pieces
Once you've made your mark, cut at a 45deg angle opposing your previous cut. Now you'll have a short piece of quarter round with a 45deg angle on each side, the insides of which will be close to the width of a floorboard. Now take a scrap little piece of wood and lay your little piece of quarter round down with the saw blade up against it and clamp the scrap behind the quarter round so that you can cut as many pieces as you need identically. Once you cut the number of pieces you need (four per coaster) at this length, flip the operation around and cut the 45 off of their other side... duplicating your first piece.