Assemble your bars, check with a square. Lay canvas on top, even it up with the bars so your one inch is about the same all around. Place one staple at the half way point on any side, make sure your staple is at the diagonal, about 45 degrees, as Baer suggested. Go directly across from your first staple, give a comfortable pull on the canvas and place your second staple. Repeat this on the remaining two sides. Check your square one more time.
Looking at the canvas, you should now see a diamond shaped design going from the four staples. Start stretching toward one of the corners. Keep your staples a few inches apart, this is important.
An amateur beginner will always have his/her staples almost on top of each other. They do this because they think the ripples in the canvas will not go away. They obviously also got one heck of a deal on staples somewhere.
If your pull is about the same, all the way around, the ripples will disappear. Anyway, work the diamond shaped ripple out of the canvas by stretching each side toward the corners. There, your done.
Oh, one other thing, use 1/4" leg staples. Using longer staples makes it real tough to remove them if you have to.
John