To set up everything completely, precisely, and perfectly. Usually relating to something mechanical or creative. The phrase comes from tuning engines. Mechanics set contact points for the electrical system of the engine and then "dial in" the timing so it will run perfectly.
If we're going to take the whole engine apart, let's get some new parts and dial it in before we put it back together.
Can also be used in other tenses and variations like:
I'm going to dial in that letter to my landlord before I send it out.
That bike needs to be dialed in before we take it to the track.
I was dialing it in yesterday so that it would be ready for the trip.