I am installing a new toilet on a new floor. I understand the flange needs to be set so that it sits on the finished tile floor. So with that in mind...

1. How do I attach the new ABS flange to the existing ABS drain? The flange does insert into the drain by about 3-4 inches.
2. How do I attach the flange to the subfloor? Do I drill through the finished tile and hardibacker, through to the subfloor? If so, what sort of large screw do I use? Do I need a large washer to sit on the flange? How many of these screws or similar should I use?

Sincerely, (with a deficit of plumbing knowledge- but a quick learner)
Ann.

The flange is glued on with ABS cement. You can use a 1/4" masonry it to drill a hole on two or three holes in the flange through to the sub floor and secure the new flange with wood screws. Good luck, Tom

To attach the abs flange to abs pipe, you need abs cleaner and abs glue, then just clean the flange and the pipe, then glue the flange and pipe and slide it together,make sure the toilet anchor bolt slots are line up even with the center of flange from back wall. Then,, yes, you need to drill the tile where every anchoring hole is. Drill them all. Then you can just use regular wood screws(at least two inches long, but I would use 2 1/2 to be safe, all depends on the overall thickness of floor(from top of flange to underside of floor)) I don't suggest washers. IF the flange anchor holes are to big for a normal wookscrew, and the head of the screw simply goes through the hole, then I suggest getting some stainless 5/16x2 1/2 screws and using them. And as I already mentioned, use all of the holes in the flange for anchoring, if there are four, use four screws, if there are six, use six screws. Hope this helps. Lee.

I plan to use the template that came with the toilet to determine the location of the anchor bolt holes. Incidentally should I drill these holes through the finished floor tile?

Use a spear pointed glass drill bit to drill through ceramic tile. The drill bits don't last very long but the work quite well on tile. Use a #12 or #14 flat head sheet metal screw. Stainless steel is preferable. Drill bits available at Home Depot and Lowe's. If you use a masonry bit you will have to break the glaze on the tile with some kind of sharp pointed object. You risk cracking a tile. AS MGD said clean and glue the flange and pipe. Make sure that the slots for the toilet hold down bolts are parallel to the back wall. Then drill through the screw holes in the flange. Drill through the tile with glass drill bit then drive screws.