If you have homogenated cells you can use differential centrifugation. It’s easy, and gives good results if you measure very exactly. If you sentrifugate the cells for 10 minutes with 1,000 g you should get pellet rich in nuclei and cellular debris. Another 20 minutes with 20,000 g and you should have pellet rich in mithocondria (and chloroplasts if you are talking about plant cells.) Then, extract the mithochondria carefully with a syringe or a vacuum-pipe, and you should have the mithocondria pretty isolated.