I use a tall spaghetti jar with a heater and a rigid length of airline tubing with an airstone on the end.
I just use 1.5L of water to around 1-2 tablespoons of plain table salt. I also add a fairly large pinch of baking soda to buffer the water up.
Then I just have my airstone on full and around 18-24 hours later I can harvest. Seem to get a good hatch rate as the eggs I purchased have an estimated 85% hatch rate anyway.
To harvest, I turn off my airstone, put a torch on the spot I want the BBS to congregate and after five or so minutes, I use a small siphon to siphon out the BBS into a coffee filter for straining.