I have a Mesa Boogie Dual Rectifier head & I need to know how to connect it to a Rocktron Patchmate Loop 8 and a Rocktron Expression Digital Guitar effects processor.

In the back of the amp there are jacks (holes) that you plug one end of a cord into. You want those holes to be "output" jacks, so that what goes through the amp winds up in the effects processor.

Plug the other end of the cord(s) into the "input" holes of the effects processor, and sometimes you need a flashlight to read which holes are inputs and outputs…

(Usually, it’s fairly obvious because the input and output holes are "labeled or marked" as to what their purpose is…just think "direction of signal"…in other words…the guitar is the first signal outputter which then goes to the pedals and/or amps which then go to everything else…)
Guitar>>>>>(foot pedal optional)>>>>Amp>>>>>Effects>>>>recording device…