This is how I learnt to stretch a canvas, a print or any painting, I hope it is helpful for you.
If you have any questions please feel free to contact me!
When stretching a canvas in the "gallery wrap" style (staples in the back) we need to make sure we have enough fabric to wrap around the borders.
Start by doing the longest sides, set around 5 staples at the center of the stretcher, then stretch the opposite side and set another 5 staples ( across from the first ones), then proceed to do the smaller sides of the stretcher by stretching and stapling right at the center.
Keep stapling side by side around 5 to 6 staples at a time and always doing the opposite side next until you reach the corners.
Fold the corner like an envelope and staple to keep it flat. Make sure both corners on one side are turned facing the same way.
*For better presentation when canvas is stretched, trim extra fabric and cover staples with white duct tape.