To stretch canvas you will need a staple gun, stretching pliers and stretcher bars (sometimes called stretcher strips) with which to build the stretcher frame.  Stretcher bars are made of light weight wood and come in a variety of lengths to allow for different sizes of canvases.  They are assembled using a slot and tenon system.

To make a frame for stretching, fit the slot and tenon against each other and push them together to form  a corner.  When the corners are fitted you will find there is enough room in the slot for the  insertion of the "key".

The keys are small wooden triangles.   Wedge two into each slot  in each corner to expand and tighten the corners.

Some framers prefer to  insert the keys after the canvas is stretched.   You may want to use a mallet  to tap the keys in.

Lay the stretcher frame on the canvas and use scissors to cut the around it.  The canvas should be cut large enough that it can fold over the frame along each side.  Apply three staples in the middle of one of the long sides.  This will hold it as you stretch it over to the other side.

Use the stretching pliers to pull the canvas taut and apply three staples on the opposite side.  Again, apply them in the middle, close together.

On a third side apply only one or two staples.

Repeat for the remaining side.  Initial application will find three staples in the middle of the long sides, two in the middle of the short sides.

On a long side, apply a row of staples from the middle to the corner.  Half the canvas will be stapled on that side.  Next, turn to the diagonally opposite side and work to the opposite corner.

Repeat for the remaining two sides.  Half the canvas will be stapled.

Returning to the first long side, apply the remaining staples from the middle to the corner.  Repeat for the remaining sides in the same sequence.  Fold the corners.

Make two folds at each corner, pull one fold over the other, and staple the canvas to the frame.

Make the remaining fold parallel to the edge of the stretcher, squaring off the corner, and then staple.

The staples are placed close together.  On larger frames a cross bar is placed in the middle to provide extra support.

Wet the canvas and allow it to dry.  This helps to decrimp  and straighten out the canvas preparatory to "sizing."

A good taut canvas is ready for "sizing", which is the  application of glue size to protect it from the oxidizing  effects of oil paint or prime.