Lets see if I understand what you mean by what's in bold and underlined.
If you want to be able to use 3 holes as opposed to just the first one (which makes it a single size belt), you have to use the entire length of the buckle end which means you have to cut out the entire horizontal prong slit. While the buckle end has 5 pairs of holes, it has 3 functioning sets (each set being what the arrows point to). The opposite end, that gets fastened, has 5 holes. The two unmarked holes on it are considered decorative. They can be used, however, if you want to retain the symmetrical look once the belt is fastened according to which set/hole you'll use, you should not use the unmarked holes. And you really should actually cut them off so the fastening end is not visible past the buckle, like V's.
To determine where you'll cut the extra length of the fastening length (if you want to actually cut it), look at the vertical line on the pattern that divides the belt and tapered end and mark/visualize it on the belt. This line should meet the edge of the buckle's inner square. Fasten the belt on the first hole to determine how much is sticking beyond the buckle. It should be just over the third hole. Cut the fastening end there.
I think I'm over complicating this trying to explain it because this part mostly is determined by the size of the holes/rivets and belt thickness, so it's reasonable to find it complicated. But this really is easier done than said! And if you don't want to further confuse yourself, you can settle for a single size belt, also like V's. If you settle for single size, then you only need one set of screw rivet holes. You don't have to cut out the pair of holes (that would've been for set 2) between the holes for set 1. As for the horizontal prong slit, the length for a single set should be 1.5" total if you go by what is on the pattern. Here, you can use the center pair of holes (between the holes used for set 1) to determine where you'll cut. It's .75"/1.905cm to the left and to the right.
If you still have trouble, the moment you have your rivets, know that the rivets should be placed in the middle of the buckle's rectangle. Slide the buckle end through the buckle so the vertical line on this side meets with the buckle's inner square edge. Wrap it over the center bar as tight as you can but of course leaving a little wiggle room. Pinch the buckle end in the middle of the buckle rectangle with your thumb/index fingers and that is where you'll cut the holes (for set 1); where your fingers meet. If you'd like making it adjustable, make a pair of holes between the holes for set 1 and determine the distance between pairs. Then make the other pairs of holes. This really is the way you should do it to be on the safe side as it'll determine the distance between pairs of holes, more or less, needed for your belt's thickness and rivet post diameter.
Last thing, the prong on your buckle is thin which means you'll have to make the horizontal prong slit on the buckle end thinner.
Oh yeah. I just noticed that your buckle prong is also kind of long. It may give you a hard time fastening the belt properly, maybe make it impossible to fasten. :|
