Do not remove the canvas from the wood stretchers. If your canvas is not stretched, you do not need to worry about any wood stretchers.
Measure the thickness of your canvas. This way you can ask a sales assistant to help you find a frame that will fit your canvas.
Buy a frame that is deep if you have a thick canvas. You want the canvas to be able to sit in the frame. If your canvas is not thick, then you do not have to get such a deep frame.
Slip the canvas in the back of the frame, as you would do for any other picture. Place a piece of paper in the back of the frame once the canvas is in it. This will help protect your painting from dust and give it a professional look.
Use clips or clamps to secure the canvas in the frame. You can buy any of these at an art store or craft store.