The most cost effective means to purchase good quality canvas is in bulk on a roll. Assuming this is what will be used, the first step is to cut off the quantity of cloth required. Do this by assembling the stretcher bars, place the assembled frame on the canvas and draw a straight line approximately 40 mm wider than the frame on each side. This is based on using a standard stretcher bar that is about 15 mm deep, and gives sufficient canvas to fold over the stretcher bars but not so much that you have to cope with great folds of material at the corners. Cut the cloth about 5 mm outside the line (it will be trimmed down more carefully after the first application of primer).
Using your preferred primer (a universal primer is sufficient, providing a good key with good flexibility for both acrylics and oils), apply a single coat firstly to one side then, when that's dry, the other side of the canvas. Make certain you go to right to the edge, covering the line previously drawn. Once both sides are dry, trim the canvas to the line, this will all help stop the edges fraying. I also prefer to use pinking sheers for the final cut to help with this.
Canvas has a front and a back, check you have the front (the side on which you will paint) face down on a flat clean surface and place the assembled stretcher in it's centre, so that there is 40 mm additional canvas on each side. Make sure that the reverse or 'flat' side of the stretcher frame faces up, with the 'lipped' side on the canvas. Starting with one of the longer sides, fold the canvas over onto the back of the stretcher as tightly as possible, but making sure that the opposite side keeps it's 40 mm excess. Pin or staple the canvas at the centre then, whilst holding the canvas taught, approximately 40 mm from each end, leaving the corner free. When that is completed, fix the canvas in between, so that there is a pin or staple more or less every 30 - 50 mm.
Begining at the centre of the opposite side, use specialist canvas stretchers to pull the canvas over the bar. Whilst keeping this as tight as possible, pin or staple the centre point of the canvas, then 40 mm from either end, and then at points every 30-40 mm between. Now the canvas is attached across the longest sides, the same can be done to the shorter sides, pulling as out as many wrinkles as possible.
At this point, all sides are fixed, the canvas is tight over the frame, with the corners still free. To fix the corners, pull the centre point of the canvas corner over onto the back of the frame, so that it is in line with the join (the point of the corner should face inward inline with the centre of the joint). Now turn up over one of the loose sides onto the corner, followed by the other side on top of that. As all these are held in position, apply a single pin or staple to hold the folds in place (see attached photograph). This then is repeated on each corner. When finished, insert the corner wedges to tighten the canvas further and remove any minor wrinkles.