Dead easy... provided you get the right trackball/spinner (in other words, if you bought a Happ/Vortex cable, buy a Happ trackball and a Vortex spinner). You'll have two cables to the harness--one for the TB, one for the spinner. Plug and go.
However, I do recall being confused about how to plug one of them in, but I don't remember what the specific problem was... so there may be some confusion. Bleah, curse my fading memory. It worked out fine, though.
-->VPutz