The Best way to stretch a canvas is to place the canvas flat on a table, put the stretcher on top and then fold over the canvas over the edges of the wood. Start by stapling the top left half and then the right bottom half, turn the canvas 90 degrees and repeat. Then do the same with the other two corners, turning it again 90 degree and then staple the last two half sides.

This works every time, but as they say practise makes perfect
Kristina Willman
Art Attack