Navigation requires that you know where you are relative to your destination, and how to negotiate the route.
To do this in space, knowing your attitude is as crucial as knowing your position, so the first thing to do is find and track the sun and a known, conspicuous, distant star.
Sirius is a good example, but it lies relatively near the celestial equator, so sometimes the sun gets in the way.
Canopus is better: almost as bright, and far south in the celestial sphere, well away from the sun. From the positions of such stars and the sun, you can calculate your attitude, and can locate other bodies by radar, data from mission control, or visual observation.
Gyroscopes can then damp oscillations and detect minute changes in attitude, while Doppler measurements let you calculate your velocity.
In space, knowing your trajectory relative to major masses in the solar system permits you the luxury of navigating by dead reckoning for millions of miles.
Only when you apply thrust to adjust your course, get close enough to bump into things, or need to maneuver into a precise orbit is it necessary to check on your exact situation and make corrections.
The Apollo missions depended on ground-based radar, which could determine their position and range, plus, using Doppler measurements, their radial velocity.
Course changes were computed on the ground and radioed to the crew. The figures were then punched into the on-board computer, which took care of the engine burns.
As a back-up, the crews were trained to be able to take star readings and calculate course changes for themselves.
This was never required, although for one course adjustment on Apollo 13, the computer was not available and the burn had to be controlled manually.
