Draw a rough picture of the shape the tile needs to be using pencil and paper. Keep the drawing as proportional as you can.
Measure all of the straight sides of the tile using a measuring tape and label them on your drawing.
Mark every inch on the side opposite the irregular curve using a pencil and the measuring tape.
Measure from each inch mark across the tile to the curved, irregular edge using a metal ruler and mark the distance across the tile for each inch mark on the paper. This gives you the measurements necessary to cut the appropriate length on each side.