You can glue down canvas before painting or after the painting is dry.
How to Glue Down Canvas for Painting
I use the common PVA (Polly vinyl acetone) water based, white, wood work glue
Gluing down canvas to small boards is an easy way to make your own art panels
You can glue a canvas down after you paint on it by using this same method.
In Australia we have a board called Masonite which is ok to use the shinny side for gluing to. Most compressed hardboards can be used and plywood is ok also.It is not a good idea to make large panels as they tend to warp and a stretcher frame type of canvas is light and more suitable for the bigger paintings.
I do not like to make glued down panels wider than about 600 mm or 2 foot.
If you are unsure what to do then you can watch this short video.
There are a few points to remember.
Lay your board face up on newspaper to catch any drips.
Cut the canvas slightly bigger than the board.
Put plenty of glue on the board.
Keep the grain of the canvas straight with the board.
Some canvas will ripple when it first touches the glue but will
lay flat after the glue gets on the entire surface.
Some canvas will shrink so better trim it when dry.
Complete training DVDs are available at PaintWithLen/shop
by Len Hend