But how do you get the first stage back?

A 1993 NASA study (TP-3335) considered the 2STO case
where 47% of the ascent propellant was carried by the booster,
which had a fuel fraction of .874.

It staged at T+105 seconds at 83,175' and 2,900 ft/sec.
Even so, it was then only 10.5 nautical miles laterally from the pad.
A glide return was perfectly feasible.