The photo shows the lower PCB, detached, with a screwdriver pointing to the 9v negative post on the power jack. Connect a fairly heavy wire to that and route it to your power switch(es). At some point, you will also need to connect that wire to the grounds of your In/Out jacks.

In the photo, I have snipped (not pried) off the top of the 9v jack, which let me peel back the inner strip inside. Be careful not to pull the jacks free from the PCB when you are peeling away the plastic on them. Incidentally, the strip is the positive (+) connection. While some people might prefer to solder directly to the PCB, I usually don't go that route. The jack is soldered to the PCB, so I piggy-back onto what's already in place. In general, I solder as little as possible onto the PCB, and not just because I'm in a hurry to actually play guitar. You can easily fry some sensitive components with the soldering iron, so the less it meets up with the PCB, the better.