I do my coating usually outside in my building but sometimes I have to do it in the house if the humidity is too high an bake on the back porch, I like the humidity to be around 40% or lower as it helps create static when the air is dry.
I start off with a container like this one, as I've found it works the best for me. They are just recycled #5 plastic containers and lids that we get takeout in from the Chinese restaurant, or you can just pick some up at the restaurant for about a quarter each.
I filled the container about 1" deep with these plastic beads, you can get these at any Walmart or craft store cheap. You can wash them off and reuse again and again if you want to switch colors, or just buy enough for all your tubes.
I put about a 1/2 tsp of powder in with the beads and shake it up for about 30 seconds to get everything mixed well, you may need to add more powder, but I only add about half as much 1/4 tsp as too much can make the powder clump on the bullets, you just want a fine coat. As you can see, the powder is already clinging to the plastic bowl.
I hold the container like this in my hand and swirl everything around for about a minute, sometimes less. Once coated, I stand them up on the base on parchment paper and bake them for 20 min. at 400 degrees.
And if all goes well, I end up with this, which it usually does. Powders that I get excellent coverage with are Smoke Yellow Green, Traffic Purple, Carolina Blue, Clear Coat, I also like Eastwood's Maroon Red and Medium Green.