In test tube how does the sperm of a man and the ovum of a woman mix together to form a baby?

Two ways. Ivf is fertilization outside body. We do not use test tube but rather plastic dishes. We sometimes simply place the washed sperm cells in the dish with the egg (conventional insemination). Sometimes the sperm is the fertility problem and we can insert a single sperm directly into the egg using a microscope and tiny glass needle (icsi - intracytoplasmic sperm injection). Only about 25% -> embryos.