How to fix a hole in the floor - subfloor repair DIY

We show every step to repair a big hole in the subfloor. All done from the top side! It's pretty easy when you know how.
We removed an old chimney and that left a big hole in the living room floor!
We filmed step-by-step repairing that big hole and it looks great.
Next step will be patching the wall-to-wall carpet, but that will be another video!
Tools and supplies:
Replacement piece of flooring (subfloor/chipboard/OSB in our case)
Tape measure
Drill
Screws
Screw drill bit
Drill bit (large enough to make a hole to put the rope through)
Felt marker/pen/pencil
Jigsaw
A couple roofing nails (or tacks or short nails)
Hammer
2 feet of rope
A couple short boards
A piece of cardboard