The slots for the advance are made shorter, usually by welding a portion closed. This will reduce the total mechanical advance so you can use more initial advance without having too much total advance. A quick check of the amount of mechanical advance can be done with a degree wheel on the dist rotor and the shaft held from turning in a vice. Turn the rotor till it stops and measure the number of degrees it turned, then double that number to get advance in crankshaft degrees. Example, 10-degrees measured at the dist is 20-degrees at the crank. Anyhow, that is how the total mechanical advance is set. The next step is to use lighter advance springs, so the rate of advance can be set. This will depend on the engine, converter, gearing, vehicle weight, etc. You can mix different springs to get the rate you need. The last part is the vacuum advance. There should be an advance number stamped on the arm coming out of the vacuum canister telling how many degrees it can add. This can be changed my modifying stop on the arm, but usually nobody bothers messing with it because the spring tension of the vacuum diphram can be adjusted with a hex wrench through the nipple of the canister. This can be adjusted to the vacuum level you want to have the vacuum advance come in at. A might-vac vacuum tester is nice to have when adjusting this.
