Just start with a linear mix (i.e. PGM. MIX or P-MIX) mixing rudder to elevator and rudder to aileron. depending on how severe the coupling is, start at maybe 5% mix, or maybe even 10%. Have a spotter with you to write down what changes it needs. Remember that the right rudder knife edge and left rudder knife edge may need different mix values, so check each direction. From there, it's just a matter of iterative tests and appropriate changes on the ground (or in the air if you feel like programming your radio in flight). Try to keep all tests at same speed (usually cruise speed) and at level altitude. I find it easier to mix roll out first, then deal with elevator, because when the plane is rolling out of knife edge it is difficult to see subtle pitching.

Now after all that, keep in mind that the coupling may be different on different lines other than knife edge (horizontal wings level, for one). Sometimes knife edge isn't the best neutral mix point, but it's an excellent starting point.

Hope this helps,
Baron