I like to use 3 teaspoons per gallon of water (2 being the min. and 4 being the max.). Mix in an old milk jug and pour into a quart spray bottle if you are doing small numbers or in a garden sprayer if doing large numbers. A quart will do about 30 deep frames. A 1/4 pound will last the hobby person many years.
Whatever you mix up must be used in 3 days, spray it on your drawn combs and anything left over can be sprayed on your veggie garden, fruit trees or flowers. The mix will go putrid after 3 days and the spores will not be viable anymore. The powder itself has a shelf life of many many years. Harmless to bees and humans and is organic also.
All you need is a good misting on the combs, trying to get it into the bottoms of the cells and any nucks & crannies where a wax worm can get a foot hold. Let them dry before you put them in storage or they will tend to mold from the dampness. Never did like garbage bags myself but they will work. I like to use a telescoping outer cover turned upside down on the floor, a sheet of newspaper and then stack the supers up with a sheet of newspaper between each super and then top off with another cover or piece of plywood. I have found that the newspaper will act as a barrier to keep a wax worm or moth from moving from one super to the next if they do live, or act like a gasket to some extent to help seal any imperfections in the box edges.
After treatment and the solution drys in the combs, the spores will be viable for many years. I had four drawn comb supers stored in my mothers basement garage that I had forgotten about for 20 years, looked as good as the day I put them away.
Use of BT on drawn combs in your swarm traps is "A#1" also.
Now for the sad news............in the USA it is not labeled for use on honeycomb (doesn't say you can't). Many years ago it was sold as Certan or B401 for this very purpose. Still available and listed for this very use in Canada (can't remember what catalog I saw it in). Not sure why they will not get it listed again for the USA, must cost out the ying yang for that. But anyway, it most definitely does work!!
Disclaimer.... I am not suggesting that you use this product in any other way than what it is labeled for.