Well, I thought it was easy. Lay down a ring of plumbing putty and screw
the drain back in place. Wipe excess away.
But it leaks after a few [days to weeks] when my daughter fills the tub. I
finally just laid a bead around the installed drain and keep pushing it back
I've been tempted to apply silicone something-or-other and hope that works,
but there has to be a real solution. Now that she's off at college, I have
a chance to attack it without interruption.