First you paint a small watercolor painting, say "8 1/2" x 11". Scan it and you may want to edit it a bit too. I used the free program Picassa 3 to crop,straighten and edit my scans. Then you join Costco Photo Center and upload your scan of your painting. You choose from the sizes they offer and also whether you want your canvas to be wrapped around wood or made into a poster or something else they offer. You pay online (but your piece is returnable if you don't like it) and they call you when its ready.
You pick it up at Costco where its wrapped carefully and waiting for you. Than you take it home and hang it on your walk.