I grabbed three paint colors, teal, yellow and pink and painted the entire canvas in messy splotches. Of course I didn't take a photo.
While that was drying, I cut out the sentiment using shelf liner that I purchased from Walmart to create a mask. (The sentiment was a Design Space freebie.)
Once the mask was adhered to the canvas, I painted a couple of coats of white acrylic paint over the sentiment and all over the canvas. I didn't want to use my more expensive vinyl, but I definitely learned my lesson. The shelf liner was very thin and didn't stick well to the canvas. I burnished it as much as I could, but it still allowed lots of paint seepage.
I ended up using a Sharpie to outline the sentiment.
My first attempt at a messy canvas wasn't what I hoped, but it was a learning experience. This is definitely something I will do again. In fact, I think this will be a great project with my granddaughter and glad I experimented first.