Timing marks.

No, you don't have a grasp on it, or you would not be asking this question.

Use the zero mark on the timing cover as zero. Self explanatory. Then, adjust the timing lining up the marks on the balancer with the zero mark on the timing cover. Again, self explanatory.

In other words, With the engine at idle, disconnect the vacuum hose at the distributor and block it off.

With your timing light, point it at the timing cover where the zero mark is and see what numbers on the balancer line up with the zero on the timing cover. This will be your initial timing setting.