The circuit you want to mod is the one with the #7 fuse for the ignition coils (under dash fuse block). You already knew that, right?
You can do this with 3 wires, some spade connectors and a normally closed momentary switch.
You're going to modify the #7 circuit, where pushing the button acts as though you've effectively pulled out the #7 fuse (as long as you're holding the button).
What I did was:
1) Made up a pair of wires with soldered on spade ends (male) that were the same size as the fuse spades.
2) Pulled the #7 fuse and inserted both of the wires with male spades into the fusebox slots where the fuse was.
3) Connected the fuse to the end of one of my wires (added a female spade connector to the wire).
4) Connected the momentary switch to the end of the
other wire that's inserted in the fuse block.
5) The
third wire goes between the other side of the momentary switch to the other side of the fuse (another female spade connector).
Locate the switch somewhere handy (gear shift), tape up all the connections, and presto! +15 MPG.
I've read there are "add a circuit" kits you can buy that save you having to make up your own spade/wire connections, and includes a handy slot to house the fuse, but didn't look into that.