1. Pre-Dive briefing
It all starts during a pre-dive briefing. Often the dive company will lead this briefing, but sometimes you need to do this together with your buddy.
Agree on the dive site, and the routing to follow underwater. Check where you will enter the water (and how), and where you will exit the water again.
Agree on a total dive time, and the maximum depth during the dive.
Agree if you will make a safety stop at the end of your dive. (You should!)
Discuss main communication signals to be used underwater; don't forget to include those of cool fish you possibly can encounter underwater at this spot!
Discuss and agree on the emergency procedures; what to do if you get separated, or in case of any physical problems. This step is essential for your own safety and your buddy.
After the briefing, you are ready to put on your dive gear. When you are done, find your buddy and do the buddy check.