I abhor food waste... So, when I finally started growing enough tomatoes to can, the sight of compost bin bound skins made me weep. This year, I ran across a post from Serious Eats that offered microwave instructions for drying tomato skins. Excellent! I loved the idea but considering the amounts of tomato skins I'd be drying at a time, their microwave method wasn't practical. So, I dried my skins in the oven just like I'd dry chilies or herbs. Keep the powder in your spice rack to top popcorn, french fries, or mozzarella.
It's simple but takes a few hours in the oven. But if you're looking for quicker results, check out this microwave method: http://www.seriouseats.com/recipes/2015/08/tomato-powder-from-tomato-skins.html
Heat oven to 200F.
Lay out the skins on cookie sheets lined with parchment or silicone baking sheets. If you don't use parchment or silicone, the skins (especially the thinner ones) will stick a bit. Lay the skins out as flat as possible. This will reduce the drying time.
Dry for 2-4 hours. They really aren't going to burn at 200 degrees. So, it's ok to just forget about them and go about your business. The skins are properly dried when you can crumble them up in your hand. If they have the texture of fruit leather (Fruit Roll-ups), they need to dry longer.
Process the skins in a spice/coffee grinder, food processor, or blender. Using a spice grinder will yield a more fine powder whereas a food processor or blender will yield a course texture that is similar to crushed red chili flakes.