<< I'm just looking for any tips or obvious things I should know about doing
Don't just butcher it out, use a little finesse and save yourself some hassles.
A pro carpenter does it this way: rent or buy a saber (jig) saw. Grind or break
off a blade so that it will cut just the thickness of the plywood. Look for the
nail patterns indicating joist location. Make a couple of parallel cuts at the
center of the joists, then two right angle cuts at the ends of those. Lift out
the square of plywood and then plan and execute further cuts in this fashion
until you have the entire bad area removed. Your final cuts need to be straight
and centered over supporting joist. You will need to glue or nail in bridging
members as necessary so that all four edges of the new plywood are supported.
Sounds complicated, but in reality it will take you under an hour. HTH