Be very, very careful! Seriously, there are several very useful safety guides around:

• Ohio-State's Electrical Safety Document
• Paul Ruby's First Power Up
• Randall Aiken's Safety Tips for Working On Tube Amplifiers
• R. Aspen Pittman's Survival Tips for Tube Amps
• R.G. Keen's Musical Instrument Tube Amp Building, Maintaining and Modifying FAQ

Please read the all the safety guidelines before you start modding.

Learning How To Read A Schematic Diagram, will greatly assist you in identifying the components and circuits you wish to modify inside any amp or pedal.

Learning How to Solder can save time spent troubleshooting later, and save parts!

Learning How To Drain The Filter Caps can help save your life! I clip the leads on the resistor to my DMM probes

Learning How To Build And Use A Simple Light Bulb Current Limiter can help save your amp after modding!

Learning the basics of electronic theory will help you comprehend how these things work and a lot more!

Trinity Amps Builders Guide
A well organized guide for tube amps!

Introduction to Basic Electronics, The Primers:
Intoductory Ohm's Law
Beginner's Guide to Basic Electronics

Electronics Tutorials:
http://www.hobbyprojects.com/tutorial.html
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
http://www.phy.davidson.edu/instrumentation/NEETS.htm
http://www.tpub.com/content/neets/14178/
MIT EE Course
The Crowhurst Files:
http://www.tubebooks.org/Books/crowhurst_basic_1.pdf
http://www.tubebooks.org/Books/crowhurst_basic_2.pdf
http://www.tubebooks.org/Books/crowhurst_basic_3.pdf