Many sources suggest scuffing the smooth side with steel wool or sandpaper, you may want to do that but I had good results without it. I did wash the plastic thoroughly with simple green. You can also use dish soap that doesn't have hand softeners,
Step 2: Paint It
TechShop has a nice paint booth with ventilation, paint the plastic with a couple of light coats to minimize chances of runs and defects. I painted the inside first so I could flip it without marring the paint. Allow about 10 minutes between coats.
Step 3: Allow To Dry
Let it dry for 30 minutes to an hour before touching it. Now you're pretty much done!