How sophisticated do you want this to be? (That's techie-talk for "How much money do you have?")

The simplest way is to open the connection between the premp output and the power amp input. The preamp feeds the 'Send' jack, and the 'Return' jack goes to the power amp.

Refinements (expensive complexity) go up from there. Like buffering the signal jacks to impedance-match the amp circuits. And selcting Pre-EQ or Post-EQ insertion points for the effects.

Bottom line?... Sit down with a good tech and do some planning over a schematic drawing of your amp. That reduces the possibility of frying your amp or yourself.