First, get a degree wheel or make one. You can download one for free, print it out and glue it to a wood disc. Make a pointer - I use a large paper clip attached to a cylinder fin with one of those big metal spring steel paper clips. Remove the muffler from the engine. Rotate crank so piston is at bottom of stroke. Insert a popsicle stick into the exhaust port and hold it up against the top of the port. Rotate the crank until it contacts the popsicle stick - that's your piston stop. Rotate the crank the other way until the piston again contacts the popsicle stick. Halfway between your two degree readings is BDC. Add 180 degrees to that and you have TDC. Accurate, and it costs you zip.

http://www.tavia.com/free_degree_wheel.html