How I used to do it
For the Boeings I've flown:
First we would set the rudder trim to "neutral" at first trim point
1. At 250KIAS
2. Ensure symmetrical power and fuel balanced
3. Hold wings level and use rudder trim to hold a constant heading
4. Don't touch the rudder trim again!
5. Record aileron trim to keep level wings
Repeat trim point at desired speeds and configurations