Because I’m placing a 5 foot tub in a room that’s 5 feet wide, this was the most complex aspect of this project. The new drains must align with the new tubs, drain connections. In addition, the new tub’s did not fall within the existing concrete hole in the ground. So I taped down a piece of cardboard, place the tub and marked the exact spot where the drain has to be.
I cut off the old tub drain plumbing just below the junction of the old joints/connectors with a hand held PVC cutter that acts like a pair of plyers with a blade. This would fit down into the existing hole so the pipe could be cut level.
Of note, this drain had typically exhibited drainage issues during the years before this project. What I found in the drain was 40 years of crud, mostly hair and what looked like dirt, and it was almost solidly blocked. It looked so bad, I was briefly worried that that drain pipe had cracked, allowing dirt into the pipe. This would have been a $20k nightmare for a house on a concrete slab, where the PVC drains run under the house in the dirt. Usually they excavate. 😳 Fortunately I was able to snake out this crud, and wash down the rest, then dragged in a garden hose and sprayed water down onfull blast to reassure myself that the drainage was functional.
This is why on the new drain pipe, I added a drain access on the backside, and cut an access hole in a bedroom closet wall, with a door so using a snake is easier. Because trying to run a snake though the tub drain or overflow is more difficult.

Template for tub drain
Then I used a handheld power hammer to jack out the edge of the concrete hole which you can see in comparison to the previous image of the old tub plumbing. Of note, there was a piece of reinforced rebar (going through the concrete) right where this tub drain needed to be. I’d not seen this before, it was metal wrapped with small wires, greased and encased with a platic coating, I assume to avoid corrosion in the concrete. I have a metal bade on my Sawzall (reciprocating saw) but that would not cut it. Fortunately I had hand held angle grinder with a Diamond blade from the last big project which made quick work of this.

Floor jacked out, dry fit. The white pipe to the left is a AC condensation drain that wil be reconnected, in addition to snake access.
Then I dry fit, connected the 3 pieces of the pipes for the tub drain and overflow drain, actually glued them as they are a set dimension, then dry fit the rest, placed the tub to check fit, marked thr pipes, checked the fit again, and then glued it all together.

Tub drain completed with bug barrier.
This hole in the ground is reported to be a primary avenue for bugs to get into the house, yet I saw no signs of bugs, especially termites. I read that 6” of sand can stop termites because their tunnels would collapse. So I bought a bag of sand, filled the hole partially with sand and then laid in some dense polyurethane foam sealing the edges with spray insulation foam.
I’ve note our house is built on a sandy soil, in fact they may have filled in this lot in with sand so that may help termites in general.

This was the fit which was flexible enough to screw in the tub drain.

Tub in place.