Rudder input
Strictly speaking, the slip indicator is not exactly accurate so long as it is displaced from the C of G.
Lets say you're flying an aircraft with a mile long fuselage, in a perfectly co-ordinated tight turn to the left, and of course the cockpit is half a mile forward of the C of G.
There would be a G force in the cockpit squashing you and your slip indicator to the right. ( I Think)
It's fairly negligible in most aircraft, but noticeably when trying to soar a glider in marginal soaring conditions. Rudder shold be used to position the slip indicator to slightly lag behind the direction of turn, and this is confirmed by seeing the piece of string being positioned straight on the canopy front.
This is why I tend to think of rudder as being a tool to counteract adverse aileron yaw, although in most aircraft, the slip indicator is still the most accurate way of measuring the slip caused by such yaw.
So long as one aileron is deflected down more than the other, there will be adverse aileron yaw caused by increased induced drag on that downward aileron, unless the ailerons are accurately designed differential ones which cause the upward
aileron to deflect more to compensate.
In gliders the rule is simple...pretend that the rudder is connected to the stick with an imaginary mechanism.
Left stick+left rudder, right stick+ right rudder, in relevant proportions.
If you do that correctly, the piece of string will tell you so, and that's about as basic as it gets. However once you get into larger aircraft, more complex designs, and changing airflow directions due to power plants etc, other effects take place which may either exaggerate or nullify what I've just said.
All far too technical for me...