Well if you shadow box in front of a mirror, you see you openings when you throw punches, so that helps that part. Obvious combination, jab jab cross. Then just slowly work of that, most combinations start with a jab, just when doing and trying them imagine someone standing there and making openings, mix up the angles. If you punch high, generally they block high, so then go to the body, then they block low and so forth. Oh and when trowing these punches, you should bring your hand straight back to guard, so you should have one hand to guard, even without a partner its always good to train like they can still hit you back, training outside of the ring will be what you bring into the ring