This is a simple, no-frills attenuator for guitar amplifiers (although it will probably work with any kind of amp, assuming you match up the impedance).
The L-pad is available at PartsExpress in various wattages and impedances. A good rule of them is to get an L-pad that is rated at twice the max wattage of the amp you plan to attenuate. Also, make sure you match the output impedance of you amp (8 ohms, for example).
Use mono jacks for input and output.
I'd recommend using some thicker gauge wire than the typical pedal, especially for higher-wattage attenuators. Shoot for 20 gauge to be safe, or use they type and specification of wire used in tube amp kits.
Note that the switch I used was from Radio Shack, and the poles are the opposite of most other toggle switches I've seen. This wont' make any difference in the operation of the attenuator however. Just thought I'd add the info. If you have concerns about which lugs are active on which throw, use a DMM with continuity testing to trace it out.
Here's the layout image. Click on it for a much larger version suitable for printing.