In addition to celestial navigation, there was very precise radio based ranging between tracking stations and the spacecraft. You could call it a precursor to

In addition to celestial navigation, there was very precise radio based ranging between tracking stations and the spacecraft. You could call it a precursor to GPS, where a pseudo-random noise was transmitted in the microwave signal to the spacecraft. The spacecraft sent back that PRN stream on its microwave downlink to Earth. I have forgotten how precise and accurate the tracking data was, but it was more than enough for the missions. NASA would take measurements and then uplink nav data to the guidance computers as necessary. With that and periodic celestial position fixes, there was no getting lost on the journey. "Just don't screw up a procedure or have a burn go bad and everything will be okay."