It is a PITA. The older ones are set up with the circuit board extending the length of the pedal. The tactile switch sits above that very close to both the footpad and the circuit board. You need to set some sort of enclosure above the pedal and then mount your switch to that. I used a Hot Wheel but I traded that off and it apparently met a horrific end. I think the recipient tried to test out the stompbox quality of a Hot Wheel car and smashed the 3pdt into the circuit board. I saw one that looked eerily similar to mine on the Craigslist recently in none too good condition. Oops. Try to find a better enclosure.
There are 6 wires coming off the board. Input, output, switch, LED, ground and I think the last is for 9v. You'll need to get into the input, output, and LED wire and switch those according to any true bypass schematic. My old DODs all power up with the effect on, so you can just remove the switch wires and it will work just fine.
And because I love posting this pic: