There are three options:
option 1 – Guitar goes to input one on the H9, from output, one on the H9 to the front of your amp. If you tend to run your amp clean, this will work just fine. However, if you are going to be using the distortion of your amp. It’s best to connect the H9 to the effects loop.
Option 2 – Connect your guitar directly to the amplifier. From the amp’s effects loop Send connect to input one of the H9. From output one of the H9 connect to the amps effects loop Return. This is a common configuration, but works best with time-based affects like delays or Reverb’s. I would not be using gain-based affects like distortion here.
Option 3: use H9’s pre-post mode to be able to direct signal to the front of the app when algorithms are set to PRE and direct signal to the effects loop when algorithms are set to POST. This videos explains PRE/POST mode: https://m.youtube.com/watch?v=46_-T7emyjw&list=PLhT2MsSeKiDw6X56B_cfGPhD4GeBd3MA4&index=32