Wire. you need to attach wire to the stretcher bars if your frame is made of a foam material. If that is a wooden frame - wire holders(D rings) to the frame. Distance to attach is 1/3 from the top of canvas and then pull the wire through those d-rings and make the wire tightened.
Trouble come when canvas board/panel needs to be hang - this is only need to be hang to the frame. Since canvas board is heavier than canvas of the same size frame need to be sturdier.
However a simplest trick if you live in a house and walls in your house are made from drywall and you want to hang 16*20 inch canvas without a frame - the simple office pin will work the magic. Just get those that look like the sand clock shape. In this budget way - You can hang as many paintings as you wish in one day. If there is no draft or doors near that open and close to create the wind they will hang this way 3 years easily (probably would be more if I did not have to move my art from the room). Also it is only a tiny hole left after and no need for major repair of the wall.