Measure your coffee table so you'll know what size cushion you'll need to make. You'll need to get the measurement of the length and the width, with the addition of an extra inch on all sides to allow for the seams. Also, measure from the side of the coffee table around the bottom to the other side. Add an extra 12 inches to this measurement to allow for the seams, as well as the area that will be tied together. You'll need to know this measurement to make some straps to hold the cushion in place. For example, if your coffee table is 36 inches long by 25 inches wide, you'll need two fabric panels that are 38 inches long by 27 inches wide. If the measurement around the belly of the coffee table is 30 inches, you'll need approximately 42 inches in length of fabric for the strap and 4 inches in width. The size of the coffee table will decide how many straps you'll need. An average size table should get away with two to three straps, placed an equal distance apart.
Purchase your fabric to cover the cushion at your local fabric or craft store. You should be able to find a vast selection of fabrics to choose from. Using the length and width measurement of the coffee table, purchase a piece of foam rubber to make the cushion out of. Foam rubber comes is a variety of thicknesses, so you should be able to find exactly what you need. The store will cut the foam rubber to your desired measurements.
Cut the foam rubber to the desired size, if the store didn't do it for you already. If the edges are slightly rough, don't worry about it because the fabric will hide it. Cut your fabric panels to the desired size, as well as your straps. Wipe the coffee table off with a mild detergent that will remove any slick polish finish that will make the cushions slip around.
Cut your tie strips into equal length pieces that will tie together in the center. Fold lengthwise towards the inside and sew a long seam, closing only one end. Turn the ties right-side out. Lay one fabric panel flat, with the right side facing up. Place the ties evenly spaced on both sides of the panel, with the raw end lined up with the outer edge and the finished ends facing the inside. Pin the ties in place. Lay the other panel face down upon the other panel and pin together. Sew three seams together, leaving the last one open.
Turn the fabric right-side out. You should end up with a piece of fabric that has one end open and several ties hanging off each side. Slip the foam rubber cushion inside the open end of the fabric. Turn the raw edge of the fabric towards the inside, so you end up with a clean edge. Pin together and sew the seam together finishing the cushion.
Place the cushion on top of the coffee table. Take each tie and tie together forming a knot under the coffee table. Make sure to tie each of the ties securely, so the cushion will fit snugly to the table. You now have a coffee table that's been turned into a seating area.