Its not to hard to get started actually, processing is similar to RGB once the subs are loaded into PS. Weights and mapping are more arbitrary than RGB, so subjective "suck it and see" usually works.
There are several ways to start up in PS, but I load Ha in 1st, convert to RGB, then load SII, copy and paste into the Ha red channel, load OIII, copy and paste into the Ha Blue channel . Your then left with Ha in the Green channel, presto, RGB !. This is the Hubble pallete and works most of the time, I have rarely used any other pallete.
Mostly though, I prefer Ha:SII:Ha:OIII (Ha also as a lumanence layer).
As previously, load Ha and convert to RGB, but then copy it and paste into itself and another layer is produced. Use the bottom layer as Lumanence (for sharpening etc, thats where the detail will be), set the layer blend above to "colour" and load the SII/OIII images into the upper layer as described above.
The 1st thing you notice then, is that the image is very green, due to the Ha (green) usually being far stronger than the others, thats why weighting is so abitrary, and the subjectivity in curving it just to look good.
This is by no means comprehensive, but will certainly give good results.
A totally different way here
http://www.rdelsol.net/ClippingMasks/ClippingMask.html
by Ken Crawford uses clipping masks, I havent tried it.
Ron Wodaskis Zone book has a NB section at the end too, a very worthy tomb generally.