1. download my MIDI file to his (logic) program
2. connect the midi cables to my synthesizer (where I want the music to come out from) and also connect the audio cables from my synthesizer to where? on his side?
Yes...he should have an audio mixer on some level that can handle your stereo pair of outputs. This ultimately routes thru to a mixdown device (CDR, DAT)....
3.At this point, I can hear the music, but does he now make an audio recording of all the tracks seperated or does he first mix all the tracks with the computer before he does this?
For optimum results, it's a good idea to "print" your seperate midi tracks. It will give you ultimate control for mixdown, with the ability to add VST plugs for sweetening (or not...).
4. lastely, if I want to use a sound on one of his sound boxes in place of one of the sounds coming from my keyboard, do I...turn off local on the keyboard, set the channel and patch on his sound box and then..how do we connect it for output?
again, route the audio outs to a mixer.
Leave your manuals in the bathroom. In one month you'll know your away around your equipmnent.