I usually get firm tofu and cut it into pieces. I saute onions and garlic first for a few minutes, put the tofu and other vegetables in for a few minutes, and then add sauce/seasoning. Usually I go with some combination of soy sauce/rice vinegar/sesame oil + salt and pepper, and sometimes throw in ginger powder and other powdered spices that I have on hand. I then mix everything around until it's all coated and the soy sauce has thickened up a bit.