There are so many ways to customize a canvas bag. Presently, I'm loving simple hand-drawn designs or even cheesy sayings written on them. For this DIY I highly recommend new Stained by Sharpie fabric markers (I tried some other fabric marker and did not like it). Stained by Sharpie has a brush tip and is easy to use on a rough canvas. I look forward to using these more for some other DIY projects. I bought mine at Walmart.
All you need for this project is a canvas tote/bag/pouch and a black Stained by Sharpie fabric marker. My canvas pouch came from Hobby Lobby $1.99.
My vintage Clinique cosmetic bag served as an inspiration for this design. First, I drew with a pencil (optional), then traced with a black fabric Sharpie.
You could also draw your favorite designer bag such as Chanel or Hermes Kelly Bag. Or write something cheesy like: "my other bags are Prada" or "I'm Not a Self-Important Canvas Tote".
Or design an image and do a silhouette: