The 'secret' to repairing a tear in a canvas is to do it from the back of the canvas not the front. What you need to do is carefully align the threads in the tear, then stick another bit of fabric on the back to hold it in place. The hard part is doing it neatly and for everything to lie flat.
Cut a piece of canvas so there's that's at least an inch wider than the tear. You could use heavyweight paper, but it's not as strong or flexible as fabric. If you haven't got a bit of canvas, any bit of light-colored fabric will do the job, but it shouldn't be too thin. Don't skimp and cut a narrow repair strip, as you don't want add strain on the fibers in the canvas near the tear.
Use an acid-free glue ("white" craft glue) or if you haven't got any acrylic gesso or medium also works as glue. Apply a thin, even layer of glue to the patch and place it over the tear. Avoid the temptation to apply lots of glue; it'll simply squeeze out the edges. Turn the canvas over, placing a book underneath the patch that's the same height as the stretcher bars so that bit of the canvas is supported. (Place a bit of card over the patch to protect the book from any glue.)
Check the alignment of the edges of the tear. While the glue is still wet, push any loose threads into place as much as you can with something small such as a pair of tweezers, needle, or toothpick. You may not be able to get ever bit of thread neatly arranged; those you can cut off when the glue has dried. Try to avoid getting glue on the front of the canvas. Put a bit of paper or thin card over it, then place another book on top of the repair and leave it flat to dry.
When the glue is dry, the canvas is ready for painting. If the canvas is still blank you can try to hide the tear under some additional gesso or printer. You may need a few layers, gently sanded to smooth the surface. Alternatively, hide the tear with impasto texture in the painting, or a collage item stuck over it.
If it were a tear in a valuable finished painting, it's worth getting an expert conservator to do a more refined repair.