Artist's have used rabbit skin glue for centuries to make a strong glue size when stretching canvas ready for painting.
The granules are soaked in water (1 part to 12). Like rice they will absorb water overnight. The glue is then warmed through on an indirect heat source such as in a bain-marie. This shouldn't go anywhere near boiling point, as the strength of it's molecular structure will deteriorate.
Once ready it can be applied straight away. You have a few hours before it begins to set into a jelly at room temperature. If you want to apply it as a jelly, it's easier with a palette knife but make sure you work it in well and remove excess.
Rabbit skin glue works perfectly when it is absorbed by canvas fibres, strengthening it, making it taut and sealing the pours. However too thick an application and it is liable to be the cause of problems such as cracking in paint films. It is water soluble and will require a few coats of primer over it.
Helpful Tip! Applying any kind of size to an already stretched canvas will pull the canvas taut. Be aware this could warp a weak frame, especially large ones.
If you are using a weak or thin-edge frame prepare the canvas first, then stretch it over a frame. Alternatively, use specially designed stretcher bars that are made to prevent warping. Deep edge ones are especially resilient. Bristol Fine Art provides a canvas stretching service that follows these rules.