Oh boy, I ain't a good explainer.
You have to begin with the premise that an accurate round in the gun is a combination of several issues; projectile, charge, twist and internal velocity. All in conjunction with barrel/rifle motion, comming together in harmonics.
Let's example; a given load will start at say 30grn with a max of 40grns. One way is to load 5 at 30-30.5-31-31.5-32,,,up to 40 all the while watching for pressure signs,,this can/could take up too 60-70 rounds to find one that groups well. let's say that it's 36.5,,now you go back and try 36.4 another 36.5 and 36.7,,more shooting, powder and bullets. Of course experianced loaders can narrow that down.
With a ladder you load 20 rounds each with a different charge, let's say in .4grn increments, 30.0-30.4-30.8-31.2-31.6,,,again the experianced loader can narrow it down and perhaps use smaller increments, but you still need the 20.
Now you go out to as far as you can shoot, 200-300 is best. Put up a large target and use a spotting scope because you'll need log each shot on a seperate paper at the bench. Begin shooting the spread and mark/note each shot, when you examine the complete target there will be at least one or more groups of shots in the spread that will be near each other in the horizontal plain (ya don't worry about left/right), these will represent the "range" of powder charge that is in the harmonic "node" your gun prefers for that combination, then you can go back and check/fine tune those loads.
Here's a good explanation;
http://kingfisher.0catch.com/guns/laddertest.explanation.html#Laddertest
