Amplifiers contain voltages high enough to Kill you even after the amp has been powered Off !!! If you don't have enough electronic experience to identify and discharge the Filter Capacitors which contain these Deadly voltages, take your amp to a Qualified tech. If you don't know a qualified tech in your area, post a request in the alt.guitar.amps newsgroup, the folks there will be happy to help you.
The Mod:
1. Replace the .68uF cathode bypass cap on the first gain stage with a 5uF cap.
2. Add a 5uF cap across the second stage 10K resistor. Marshall even provides the holes in the PC board to mount it.
3. Change the voltage divider to a 1 meg and 220K pair. The 1meg stays unbypassed.
4. Remove the entire tone stack and replace it with Fender values; 220pF treble, .1 bass, and .047 mid caps. Also change the slope resistor to 56k. This will make your tone controls much more responsive. You may also want to adjust the 470k/470pF network on the front end to your personal taste. Some people prefer the tone with the cap completely removed, see what you think.
Note: You should check your final product with a scope. This adds a LOT more gain to the preamp and some amps may want to oscillate, (especially if you push the front end with pedals). You can fix this by running bypass caps across the plate resistors as necessary. But try to avoid this unless it's absolutely necessary.
Finally: In these amps it's a good idea to ground reference the heaters with a pair of 100 ohm resistors as well as clean up the wiring a bit. Also put a good tight twist on the heater wires and twist the wiring from the cap cans to the board.