You have a skill saw I hope....just kidding.
You need to cut the floor with the saw set for the thickness of the floor. Use an old blade because you may hit some nails.
Find the joists underneath and mark the centers on the floor.
Then when you cut this line and remove the decayed piece, there will be room to attach the new plywood to half the joist.
If you need to support it more, install some blocks where you need them.
You'll be fine, we will be around.