What it entails is developing the "sight picture" in your mind of what the runway looks like from the air when you are 45 degrees off the threshold at the nominal turn point.
It's my own personal opinion, but I think that for a brand new student the best method is to start off using a predetermined landmark on the ground to locate yourself precisely for the turn, and concentrate on internalizing the runway sight picture you see from that location.
As quickly as you can, you start to replace dependence on the landmark with judging the sight picture of the runway, until you can ignore the landmark but get reasonably close to it by runway eyeballing alone.
You must not develop a reliance on landmarks at your local airport as a crutch, because as soon as you go to another airport you're back to square one, but you need a starting point somehow. You will need to be able to locate your turn reasonably well without depending on ground land marks before you can solo, so try to develop the correct sense as fast as possible.
One way to help with judging the 45 degree aspect of a runway with a well defined threshold edge is to study the apparent angles of the runway edge closest to you and an imaginary line extending along the threshold's edge, relative to the horizon. They should form a "level" V as shown below.