Starting in the middle as in stretching a painting canvas will do most of the work, but if you want it to stay flat when it gets damp or wet then it would help to wet the canvas before you start. This is a technique that is often used for watercolor paper so that it stays flat while painting. It will depend what you are stretching it over. Can the substrate handle the shrinking and pulling that will happen when the canvas dries. And, don't pull it too tight. Remember that if it is already wet, then it is as loose as it is going to get. So, it just needs to be taught enough (flat- but not super tight). Pulling it tight when it is wet will probably cause it to tear when it dries. But always do start in the middle and work out toward the corners.