First of all I was inspired to this by Oh so Lovely Vintage, a beautiful blog and one of my favorites! So this is what I used:
-I got almond spray paint to touch up the suitcase since it had minor scratches (you may not need this)
-2 or 3 sponge brushes
-A stencil for your polka dots (I used a tape roll)
-3 different paint colors for the dots
1. After I had spray painted the scratches on the suitcase, I let it dry over night.
2. The next day, I carefully started with one color and using my roll of tape as my stencil to make perfect circles.
3. Once my first set of circles dried, I went ahead and repeated step 2 with my next two colors.
4. Wait for the first side to dry and then work on the other.
I hope my steps are clear, or you may definitely just use the steps from the blog I found this at.