This is my simple version.
The fill holes need to be large enough to put the turkey baster, syringe tip, or git rot squeeze bottle tip into(3/16-1/4"). The breather holes only need to be about 1/8th inch.
Get Rot expands as it goes off so tape off the area before you drill and let the epoxy expand onto the tape. Tape off the breather holes as the epoxy flows out to force it to the other breather holes. Wipe the excess off before it sets, and don't worry too much about the gel coat. If you are really feeling up to it, use silly putty to make a mold of the nonskid pattern and use this to press the gel coat to make the repair look like the old stuff. Just remember that polyester gelcoat will not cure over epoxy, so you will need to make a suitable epoxy gelcoat substitute.