Before you receive a starter culture:
Prepare a shoebox-sized container of soil (including some leafy or mulchy material). I find that "real dirt" seems to work better than coconut fiber. Plain bagged "top soil" is OK, but don't use "potting soil". To prevent mites, bake the soil before using it. I use the microwave for this, but I've heard that this is dangerous, as small rocks can explode or cause the microwave to spark. A regular oven is safer (bake at 300F for an hour). A steamer would probably work too. You just need to get the material very hot all the way through.
When you receive a starter culture:
Put them into the container of damp soil. Bury a bit of bread near the worms (see below).
Feeding the worms:
Feed them whole wheat bread softened in water. Initially, feed them only a tiny bit of bread (about 1/2 inch x 1/2 inch piece). Once their numbers increase, you can increase the size of the bread bits accordingly. When the bread piece is gone, add more. If you routinely bury the bread under the soil, this will prevent massive mold problems. (If you always put the bread on top of the soil, you'll get massive amounts of blue mold.)
If you want to supplement the worms, sprinkle the bread with a vitamin/mineral supplement. I use Nekton-Rep-Color. I have no idea whether this can actually improves the nutritional content of the worms, but it certainly can't hurt.
Whiteworms are a cool-temperature critter. I'm not sure what their upper tolerance is, but they will definitely die if you let them warm above 25C (78F). Keep them at the same temperature you keep your newts and they should be fine. On the cold end, they should survive down to near-freezing, but they may reproduce quite slowly if they are kept that cold.
Harvesting the worms (for a terrestrial tank):
Just take a spoonful of worms from near the bread and put it in one corner of the terrarium. Put a TINY piece of damp bread on top of the spoonful of whiteworms. Replenish this tiny bit of bread when it disappears (and add more whiteworms if needed). In effect, you are "culturing" the whiteworms right in the terrarium with the animals. Be sure to use tiny bits of bread, so it disappears soon enough to prevent a lot of mold. Putting a bit of bread there will keep the whiteworms at the surface, where the sals can find/eat them. You can try to supplement the worms by sprinkling the bit of bread with calcium/vitamin powder.
Harvesting the worms (for an aquatic tank):
When you want to harvest the worms, put some bread on the surface of the soil, and the worms will be right there on top to scoop out. Scoop out with a spoon and rinse with water until the water is clear. (Rinsing procedure: add water, let the worms settle to the bottom, pour off most of the water.) I rinse them at least 5-10 times in a small cup. You may need to pick out the larger pieces of soil/mulch with a tweezers. Then add worms to the tank with an eyedropper. There may be some small bits of soil in with the worms you add, but this is not harmful.
An example of a whiteworm culture. A piece of bread is mostly buried.
When the bread is turned over, you can see that it is crawling with a thick layer of pure whiteworms. This layer can be harvested, relatively free of dirt.