Nancy Zieman from Sewing with Nancy recently posted about making 1/2 square and 1/4 square triangles and some block ideas. Use this link to see it:
Her little tool is neat but you can do the same thing without it. Cut a strip of cardboard 1/2" wide and trim one end into a point shape. Set it on the square and mark. Those are your sewing lines. After they're stitched, cut down the center and press open.