This maneuver, with constant heading and increased rate of descent to correct a high glide slope, is known as a "forward slip".
First, before attempting, speak with a qualified instructor familiar with the specific type of plane you are flying.
Second, practice cross controlled stalls at altitude to validate safe approach speed for that maneuver. The 172 is extremely forgiving, but this must not be done at marginally low approach speeds (if you are high, you will rarely be slow, I did mine at 65 knots).
Thirdly, be mindful that the POH does not recommend trying this at 30 degrees of flaps as a tail pitch oscillation will result, due to flap and slip induced turbulence on your tail.
With the 172, roll coupling with rudder is much less than at cruise speed, so:
You are coming in at 65 knots (trimmed), 20 degrees of flaps, throttle at idle, and looks like it will be past the first 1/3 of the runway, decision: 1. Go around 2. forward slip
If you are not too high and elect to do a forward slip in a 172, push the rudder all the way over. This would probably be lethal in a low wing, dihedral aircraft due to roll coupling, but the 172 yaws with much less roll.
You will be seeing your old aiming point moving up on your windscreen. The aileron is used to hold line with the runway, you just work it back and forth. Once your (shorter) aiming point is achieved, smoothly center ailerons and rudder first, then use pitch to break glide path and round out for flare and landing.
You should not need to stay cross controlled while applying elevator.
As far as how much aileron is needed in the maneuver, I do not recall it being close to what was available in either direction.
We want to SAFELY increase our rate of descent. Why full rudder deflection? To maximize drag while maintaining safe speed and AOA. Ailerons are used to counter the drifting and rolling tendency created by side force. This is a FORWARD SLIP. It could also be called a "flat or skid" slip. It works.
Now to consider the SIDE SLIP. Rather than increasing drag, we reduce VERTICAL lift component by rolling with ailerons. This can be done very gently to coax a plane floating in ground effect down to the runway.
Recovery from the forward slip uses the rudder, which is always functional, even when the wing is stalled.