It depends if you want it as an always on buffer or only on when the pedal is engaged.
If you want it as always on, then go from the input jack to the buffer in, then the buffer out to the 3pdt in. (Obviously wore the 9v and grounds as normal).
If you want it on only with the effect engaged, then put it between lug 1 of the switch and the PedalPCB in.