I'm a cartographer, and that's my expertise IRL. Quickly, it's pretty much this:
Longitudes and latitudes are measures of angle. Latitude is the angle measured on a plane that crosses the equator to the center of the body and then to the center of rotation (the poles). 0° is at the equator, 90° is at the poles, and positive values are north of the equator while negative values are south.
Longitude requires a reference meridian (meridians are orthodromes that pass through the poles - and orthodromes are circles that have the same radius as the body, or the equator) from which the measure is taken. Longitudes range from 0° to 180° and positive values are east of the reference meridian while negative values are west.
The trick now is to be able to use your HUD compass to know your bearing. If you're heading towards 0, you're heading north so your latitude will increase. Head to 180 and your latitude will decrease. In both cases, longitude will remain the same. If you head to 90 or 270, then longitude will increase or decrease while latitude stays the same. That's pretty much it.