Do a Google search on:
"youtube - creating voices on the yamaha s90" or
"youtube - layering voices on the yamaha S90"
You should find a fair selection there. Just about anything that applies to the S90ES will apply to the S90 Classic, as well So will most of what applies to the S70/S90XS, as long as you keep in mind that the XS models allow combining up to 8 elements (wave samples) into a voice, where as the Classic and ES models only allow 4, and the XS models have some additional effects capabilities, but the basic theory is the same. As a matter of fact, when it comes to basic voice creation/editing, this goes for just about any model in the Motif line. At the outset, it is usually best to try modifying existing preset voices and guaging those results - experiencing, first hand, what sonic effects the various wave shaping parameters have on the final sound, then moving on to creating your own voices entirely from scratch, rather than trying to learn it "all at once" with a brute force attack on the learning curve, but if time is short and you are having problems layering some very specific elements or voices for an upcoming gig, then I would recommend also posting this request over on the S90 sub-forum of "motifator.com", as those are the favorite haunts of the Motif gurus. Again, keep in mind that any "S" model is just a "stage piano" version of the equivalent Motif model, so voice editing should be exactly the same on both models. This should broaden the spectrum from which you could expect to receive experienced assistance.
Here is a link to a specific YouTube video showing how to do layers and splits on the Motif ES that you should be able to adapt to the S90. He goes through this pretty fast and he doesn't really elaborate on what he is doing or why he is doing it, so you will probably need to watch it several times, which is a problem I have with most video tutorials.
