1. Make sure to understand what each sign means as they will help you. Most notably, the "?" sign indicates that the hidden star is nearby. If you can spot those, it will help you immensely.
2. If you are stuck on a level, look it up on YouTube. There are plenty of walkthroughs to help you out.
3. If the mission doesn't have a pro medal, then you do not have to get all of the stars in one run. You can get one of them from one run, then the next run get another.