TIP. For any size larger than 18 inches on the long side, I recommend that you use heavy-duty stretcher bars. This timber is strong enough to make the traditional 'cradle' support unnecessary. However, you will still need to set-in an upright centre bar in the slots provided. It's a good idea to reinforce each end of these with a small piece of timber nailed to both upright and stretcher bar.
TIP. Keys - those little wooden wedges designed to be inserted in slots at the corners of the finished stretcher - are best not used at all. You can tack them, in a small plastic sandwich bag, to the back of the stretcher so they're handy if you do feel a need to tighten the canvas later.
TIP. Use stainless steel staples only; ordinary steel will rust and may rot the canvas. Invest in a good, heavy-duty staple gun; your hands will really appreciate it. Always cut your piece of canvas from the roll 4 inches longer each side than the stretcher. Don't trim it off at the sides. It won't look so neat, but if ever the painting needs to be de-mounted or re-stretched, the framer or conservator will bless you.
The day will come when nothing else but linen will satisfy you, and you'll be glad you're already familiar with the techniques for stretching and painting on canvas. As an organic fabric, any canvas absorbs moisture, swelling as it does so, and stretches or sags with the pressure you apply while working on it. Not a worry; that's just natural. When the work is finished and the paint layers begin to dry out, the canvas will return to its original state. There's no need for you to tighten the keys at the stretcher's corners; it's far better that you leave them alone.