You would connect the cable connected to the input of your overdrive to Channel1 of the H9. Then connect Channel1 out from the H9 to your overdrive input.

Similarly, connect the output of your overdrive to H9 Channel2 in. Then connect Channel2 out from the H9 to the cable that was plugged in to the overdrive output.

Be aware that you may encounter ground loops, so may need help beyond what we can offer.

We are working on videos to explain all this, but they have not kept up with the software.