I just did this with a bottle of my own hefeweizen, some pranqster, and one of my own apa's.
Add 50g of dme to 500ml of water and boil for a couple mins, I usually add some yeast nutrient.
Cool and add to a jar you ran some starsan through.
Shake the crap out of it with a lid on it to aerate.
Start drinking and add the dregs of a couple bottles to the jar with the wort. Cover it in some foil you sprayed with starsan.
Swirl it every once and a while when your'e in the kitchen and in ~3 days it should be enough to add to a 2l batch of wort. (200g of dme to 2l of water)
Let that go like a regular starter (foil on top, swirl everytime you think about it), chill, decant, pitch like normal.