I am trying to install a new toilet flange in a new construction home with a ceramic floor/concrete base. 1) how low do I have to cut off the existing soil stack. 2). How do I secure the new flange to the ceramic or concrete. (With a plywood subfloor, I would just use ABS glue on the joint and screw to the floor. What is the practice for ceramic/concrete.

Drill through the ile and into the concrete then use a Tapcon screw. Cut the stack flush with the floor

If you have 3" pipe, the flange will fit on the outside of the pipe. Cut the pipe off flush with the floor or just lightly below. Best tool for cutting the pipe below floor level is a "inside pipe cutter." Small rotary cutter, chucked in a drill.

Install the flange and screw to the floor. Small, spade type drill bits for drilling glass work very well for drilling through tile. They don't last long but work very well and are quite cheap.

If you have 4" pipe, the flange will fit inside of the pipe. You will have to cut pipe about 1" below floor level to accommodate the bevel of the flange.