What do you like best?
Is it waxed? If not you can try the Filson wax paste hit with a blow dryer it will melt in. Heads up it will darken the color and can rub off over time. For light rain probably be fine. Heavy rain like fishing applications I run Goretex Pro
On canoe trips we don't want our canvas packs to be water proofed, we want them to to be able to dry out. We use liner bags to keep the stuff in the packs dry. It works better than ever trying to water proof a pack, it is the way it has been done for many years for everyone. https://www.duluthpack.com/products/poly-pack-liners
If you are just looking to shed some rain I would use a silicone spray like Obenauf's Water Shield. Like Pavan said though if you water proof your pack you will lose the breathability of canvas.
As I understand this, he is referring to a shoulder bag. If the goal is to shed rain, go ahead and spray it, if there is anything that absolutely positively must stay dry, like extra socks, a sandwich, a back up underwear, or toilet paper, inside said murse, perhaps a sealable plastic bag and a package of Wet Ones to replace the toilet paper. Go top the line with the Huggies, why do those babies get all of the good stuff.