Start off by mixing out all the bad habits your plane has, if it is a pattern type plane it will have very little, if it is a stick type it will have quit a few.
I start with the roll; play with different amounts of differential until it rolls smooth and with out waging its tail.
Then I “teach” it to do flat rudder turns, most planes will tend to bank to the side you turn using rudder, so mix in reverse ailerons to keep the plain level. After you got it to do that a knife edge will not be so difficult, you may have to mix in some up or down elevator to keep the plane flying straight in a knife edge. The plain will also turn better in a stall turn with out rolling.
I also mix in throttle to elevator so that I have a bit of down elevator when I close the throttle, this helps to get straight down lines in maneuvers.
Depending on how powerful your engine is a bit of right rudder mixed with the throttle would help to keep the plane straight when climbing vertical.
Most important! The object of mixing is to make the plane fly better with les effort from your side.