A master volume allows you to run the preamp at full bore, and then limit the signal going to the power amp. This gives you preamp distortion, but has no effect on power amp distortion. So the amp doesn't sound the same as when it's being run full-out, but you will be able to get more distortion at lower volumes.
So a master volume is like a volume control for the preamp, giving you the ability to throttle down how much signal comes out and goes into the power amp. To install one, you need to break the signal line and put in a pot.
On the Marshall 1959 Super Lead schematic, the signal leaves the tone stack and enters the phase inverter at point A. This is a convenient spot to install a pre-phase inverter master volume, because it is easy to lift the wire coming off the wiper of the treble pot and insert a 1Meg pot. One side of this pot is grounded, one side attaches to the wiper of the treble pot, and the wire you unsoldered goes to the wiper.
Kevin O'Connor has a good explanation in The Ultimate Tone about how this affects the tone of the amp. When the preamp signal is attenuated, some treble is lost. The more attenuation, the more treble that is lost. A series resistor of about 100K can help reduce this effect, but you'll lose a little volume.
Another approach is the post-phase inverter master volume. This is a little more complicated, and is inserted in area B on the schematic. The signal from the 2 plates of the phase inverter tube is attenuated in this approach, and it puts the sound of the phase inverter tube under the control of the master volume. Bruce Collins has a schematic for this. It uses a dual pot to attenuate the 2 signals coming out of the phase inverter so the attenuation is the same for each line.
A third way to add a master volume uses the area between B and C on the diagram. A 1M pot is connected between the outputs of the phase inverter after the 0.022 caps. One output goes to the pot's wiper, and one outside lug of the pot goes to the other phase inverter output. (The other lug on the pot is not used.) As the resistance of the pot is decreased, the out of phase signals cancel, resulting in less drive to the power tubes. This approach is used in the Matchless Chieftain and Spitfire amps, and from what I've read, is credited to Ken Fischer as the "master volume 3 from his Trainwreck pages." I haven't read this publication to be sure.
Another method credited to Ken Fischer involves the bias splitter resistors in area C of the diagram. I'm a little unclear on how to do this one. According to Obsolete Electronics in the Marshall Tech section, these resistors are often 220K, and can be lowered to get more grind. In their Fender Tech section they say they can be removed and replaced with a dual gang pot for a master volume. From this, I figured that you'd use the pot to vary the resistance of the bais splitter resistor. But then I found these web pages that show the outside lugs of the pots being connected so as to replace the 220K resistors, and grid of the tube(s) being connected to the wiper. What concerns me here is in either case you can set the pot so there is no resistance and therefore no bias splitter in the circuit. I have a hazy memory of forgetting to include the 220K resistors in one homebrew and the tube plates started glowing red, but the amp might have been cathode biased. Anyway, proceed with this at your own risk.
These circuits sound different, and I don't have experience with the post-PI master to comment on the differences. I find the pre-PI master on my Marshall useful. The amp is noisy by itself, so with a little attenuation I can practically eliminate the hiss. And getting a little more preamp distortion sounds good at times. But the amp really is wonderful when the power tubes distort, and the master volume doesn't give you this.
Determining the location for a master volume involves determining if there is DC voltage where you plan to insert it. If you can put the master volume after a cap so it only controls the AC audio signal, it works better--DC on a pot is scratchy when the pot is rotated. In the schematic at left, the tone stack acts to block the DC, so it doesn't matter whether the pot is placed before or after the 0.022 coupling cap. It's just easier to place it before, because that's a wire that can be relocated. If you find that there's DC present in your amp, you may want to add a coupling cap to remove it. Bruce Collins adds caps in his circuit for this purpose. The Ken FIscher versions are after the 0.022 copuling caps.
Master volumes don't sound good on all amps in my opinion. I don't like them on Fenders, but I do like it on a Marshall 1959. Since you're adding a pot, think long and careful about drilling a hole. See if you can find an existing hole to use so you can restore the amp if you don't like the master volume.