Making a stretcher.

Home-made stretched canvases are much better than bought ones... better than the cheaper bought ones, that is. Cheap lightweight canvases are good for practice paintings, but if you want your art to be taken seriously, you need to use solid, reliable strong canvases... with good frames that won't warp after one year.
There are premium-grade canvases which are excellent but these are very expensive.
And the best way to get a good strong solid canvas is to make 'em yourself!
The tools will be the most expensive part, but will save you heaps in the long run.

Materials:

• Four lengths of 2 x 1" (42 x 19mm ) dressed oregan or pine timber. The length is whatever size canvas you want.
• MBF triangle corners. (see photo below)
• If you have a very big canvas (100cm or more) you will also need some bracing cross bars to stop the lengths from curving in when the canvas is pulled tight.
• Long nails and short tacks

Tools: 
saw, plane, hammer, staple gun

1. Making the stretcher

I am making a canvas 100 x 100cm.

The pre-cut lengths are not always right for me, so I have to saw the timber.

Here I have four lengths, ready to "shave". 

This means that I plane a small slope on the inside edge of each length.  Here is a diagram.

This edge is removed so that the stretched canvas sits proud, and does not touch it. 

Otherwise there will be a 'ridge' when you press with your brush

Next, nail the four lengths together. 

To make sure your stretcher is square, the two diagonal measurements have to be the same.

If they are different, squash the diagonal opposite corners to make it squarer, then measure again. 

You'll have to do this a few times. Once it's right, nail on four MBF triangles on the corners. This will hold it square.

Here is my stretcher, 

ready for covering with canvas.