Getting Started With Astrophotography

You need to follow four steps to astrophotography:
1. Focus,
2. Frame,
3. Test Exposures,
4. Start Exposure sequence.

In daylight photography all of these are done automatically for you, by the camera (except framing), but in astrophotography you need to keep the steps distinct.

You may like to see three videos which illustrate the above points in detail:

Part 1 - 

Part 2 - 

Part 3 -