Ok, welcome back! This maneuver, constant heading and losing altitude (increasing rate of descent) correcting 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 (changing AOA, you are doing an impersonation of Flipper due to flap and slip induced turbulence on your tail).
With the 172, roll coupling with rudder is much less than at cruise speed. To be honest with you, it was barely noticable, so:
You are coming in at 65 knots (trimmed), 20 degrees of flaps, throttle at idle, and: looks like its gonna be past the first 1/3, decision 1. Go around 2. forward slip
ok, never force a landing, go around!
But, 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.
Ok, throttle idle, rudder full stop, pitch set for 65 knots, you are seeing your old aiming point moving up on your windscreen. It's working! 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, the 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, well it's been a while, but I do not recall it being close to what was available in either direction.
Appreciate the more learned response from the writer considering the ENTIRE aircraft in evaluating aerodynamic forces! And we seem to be starting to speak the same language, so: Mira y Escucha, por favor.
Firstly, yes, that's what flaps are there for, no doubt. The scenario is 20 flaps in, throttle idle, 65 knots (not crazy fast), still high. Go around considered, enough runway left for safe landing attempt.
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. But if held too long, it will create a significant amount of sink. That, combined with the need to roll back out of it with ailerons, makes it slightly more dangerous. Recovery from a forward slip uses the rudder, which is always functional even when the wing is stalled.
The side slip resulted in the crash of one of the finest pilots of her age, Hanna Reitsch, when she tried to save a Komet that could not release its wheel dolly after taking off. The increased weight, combined with the interrupted airflow over the wing and change in relative wind when the sinking aircraft rolled out of its slip, caused it to stall.
So, with the forward slip, we use a lot, a little, some, about 1/2, etc. of our aileron to control and recover by centering ailerons and yawing nose back to straight with rudder. We also make sure this is done with ample speed margin and practiced at altitude first.
There are no liars here, just people sharing experiences and finding common ground.