How to DIY Picture Canvases?

I am currently doing a room makeover for my daughters

and i wanted some picture canvases for behind their beds. After looking everywhere i couldn’t find the size that i wanted for less than $90 so i decided i was going to make them myself.
I found my canvases at ROSS for $7.99 they came in a package of 2.
I printed my pictures at Walmart in matte photo paper: my picture size was 20×30 and they were around $18 each.