Measure the printed canvas to determine the size of the stretcher frame that will be needed. Make sure that you take the stretcher frame that is around 3 inches shorter from each side of the original printed canvas.
Now place the canvas out at a solid but plain surface, and then place the stretcher at the top of it. Make sure that you place it right at the centre. Now by using your hands, fold one side of the canvas and staple it from the centre. Pull the opposite side of the canvas, making sure that you pull it as tightly as you can, and then staple it from the centre.
Repeat the process to the remaining sides as well.
Once you have placed a staple at the centre of all sides, return to the first side you stapled, and pull it tightly by grabbing it. Once you are certain that you have pulled it tightly, staple it around 2 inches to the right. Pull it again, and now staple around 2 inches to the left.
Repeat the process on all the remaining sides of the canvas.
Once you are done stapling all the sides, pull one of the corner flaps tightly. Fold it over, and place a staple. Repeat the process at all the remaining flaps as well. Make sure that you have folded it after tightly pulling the flaps.
Now turn the canvas print over, and closely check for any crease or sags. If you have stapled all the sides and flaps by pulling tightly, then it is unlikely that there will be any. However, if you do find one, then adjust accordingly by pulling out the staple from that side, and repeating the process.