I can't link to some of the plans (search), but the basic idea is that you get a small tank, only fill it up to 3 or 4 inches with water, seed it with pods, and keep it very green (i.e. lots of plankton), and have just an air line bubbling away for oxygenation (no powerhead). Keep salinty and temp same as you would a regular tank. You can also include a little chaeto, sand, and a light if you want. If you want to split the culture into bottles, each with an airline and fed by drops of phyto every so often, you can do that as well. You can siphon up the little buggers into bottles to grow some more before adding to your DT to reseed their population.
The point of the offline system is both to be a complete refugium (i.e. no predation or transfer to any DT while growing) as well as being able to keep things greener than you would want in a DT.