Installing a P-90 in a Les Paul® Junior, SG® Junior, and other single-pickup guitars is perhaps the least complex wiring setup of any electronic instrument.
Reduce unwanted electrical noise by using shielded coaxial cable for your longer wiring runs (for example, the connection between the controls and the output jack). Coaxial cable contains a center "hot" conductor which is shielded by an outer braid used for the ground wire.
For short connections between components, use standard non-shielded wire. Typically, white wire is used for hot and black for ground.
Wiring two P-90s (Les Paul Special)
When using shielded coaxial wire, the outer braid is connected to ground. The inner wire is the hot conductor to carry the signal.
Alternate volume control wiring: You can wire both of the volume controls of a Les Paul (or any other dual volume control instrument) so that you can blend the volume of the pickups independently. This solves the problem in the middle position of the selector switch where turning down one volume control affects both pickups.
Pickup height adjustment
There should be about 3/32" gap between the top of the low E polepiece and the bottom of the low E string fretted at the highest (21st or 22nd) fret, and about 1/16" for the high E. P-90s can typically be adjusted closer to the strings than single-coils, since they don't have as much focused magnetic pull. Experiment with the height of your bridge and neck pickups to find the best balance in output and the best tone.