I have a old sear canvas tent what is the best thing to waterproof it for Colorado

A nylon flysheet!  

Sorry, I really don't have a clue, but I couldn't help m'self!

I've used Canvak in the past with good results.

I recently did a search for waterproofing canvas and found several recommendations for 303 Fabric Guard. I bought a gallon and will try it on my trailer awning next week. (It isn't cheap!...but if it works it will be worth it to me.) - John

I use Canvak.  Proof that it works well is my first canvas tent, purchased in 1972 by my Pop when I was a Boy Scout, just got replaced 3 years ago at age 34.  I waterproofed it every other year.

Treat em right and they'll last forever.