There are two options this step.
Installing toilet flange on wood floor.
It goes under the flange and transfers the load of the flange and toilet and you out onto more solid surrounding wood.
Next install a new wax ring.
If only the area directly below the flange is rotted you can install a two piece steel closet flange support.
In this video i demonstrate a simple and permanent method of installing a toilet where the new tile floor was installed over the original subfloor but the to.