Set piston one (closest to timing belt) to TDC. The piston will be at the top of its cycle and the white dot on the crank pulley will line up with the mark on the timing cover.
Make sure the cam gears "UP" arrows are pointing up, and the marks on the left and right of the gears line up with the dashes on the plastic cover. You may want to use a straightedge like a ruler to assist.
Put the timing belt over the gears and tighten the tensioner. Rotate the crankshaft counter clockwise a couple of times and make sure that the timing is still the same. Sometimes you may think it is correct, but after a couple turns it will be a tooth off. Adjust and turn again.