This seems an easy task, but elementary precautions are required to make sure you don’t damage your canvas.
First, you will see that you can insert the wedge in to different ways (depending on what long side of the triangular piece of wood you slide in contact with the stretcher. Personally, I prefer to slide the wedge so that the end part I will be hammering forms a right angle with the stretcher (have a look at the photograph and it will become clear).
Wegde put sideway
Wedges with the end perpendicular to the stretcher
Before hammering the wedge, insert a piece of cardboard between the fabric and the stretcher in order to protect the fabric. This is a simple way to prevent the hammer from denting or tearing away the canvas.
Piece of cardboard protects the fabric
Finally, make sure that you hammer alternatively the two wedges at each end of the same stretcher. This way the tension created by the stretcher will be parallel to the weaving and equally spreaded.