Most guys use either toilet paper or facial tissue, others use the tissue from shoe-boxes... Personally, I don't recommend the toiletpaper, as it disolves and tears too easily..
I generally use facial tissue m'self... It gives a few more wrinkles than the shoebox-type does, unless you're going really taut canvas..
Mix up a solution of 50-50 water and white glue
For rolled canvas, cut a section three times the width you want. Then fold it into the width of the roll, so you have a "rounded" look to each side of it.. Tie it with thread in the middle, and then each end.
"Paint it" with the water/glue mix and let dry thoroughly.
Then mix up the thinned paint-color you want, about 2 parts thiner, one part paint, and brush it on..
Once it's dry, then give a wash of brown or a darker shade of the base color, on the inside of the wrinkles and folds, followed-up with a dry-brushing of a lighter shade of the base color..
I recommend using enamels for the entire process, since they "penetrate" better than acrylics.
For taut canvas (like tents), same thing, just cut it to size and put it in place. It's much better to have your tent or lean-to constructed and in place before you try to paint it...
For tarps covering boxes and gear, simply cut it to size, then drape it over the gear/boxes, wet it with the water/glue mix, then push it down into place, making sure that it gets "molded" around the stuff, like real canvas does.. Then after it's dry, paint as above..
Don't forget the tie-downs...