I've been doing an internet collaboration with a drummer and two singers for going on a couple of years. The process will depend on how the work is to be divided up, and who is doing what. I've never tried co-writing. In my collaboration, I write the songs and play the guitars, bass, and keyboards. As the songwriting and arranger, I'm in the position of quarterback. Here is our work flow:
1. I write a song and record a demo version with my own vocal, programmed drums, and instruments (sometimes the full instrumentation, sometimes a stripped down version. I try, as much as possible, to establish a solid groove because the first order of business is to get the drummer something he can work with.
2. I send the drummer the following stems as .wav files: vocals only (that took nerve at first, but I'm over it!), programmed drums only, and guitars, bass and keyboards only. Separating the vocals out to their own stem is not strictly necessary but the drummer prefers it, and I understand why. I'll also send him an Mp3 of my latest demo mix for reference. I also let him know the tempo so that he can set his DAW to the same--a good practice that makes things easier.
3. The drummer composes and records a rough take of his drum parts, then send the tracks to me. I plug them into my DAW in place of the programmed drums. Then the drummer and I do a little discussion back and forth as to what we like and don't like about the take. The point is to get the groove as solid as possible. Communication at this stage is important, as well as a willingness to be flexible and negotiate as two people will not necessary hear the song in the same way, though at the end of the day both need to be happy with the result. After all that, the drummer will send me a second take which is usually the final one.
4. I import the final drum take into my DAW. Usually, at that point, I will retrack the bass line to get it tight with the drums. I will either retrack or do manual edits to the other instrumental tracks to get those sounding tight as well. I want it to sound like the drums are driving the arrangement, not following it. If I did not record a complete arrangement before sending the tracks to the drummer, I'll complete it at this stage, recording whatever additional tracks the song needs. Typically, I'll retrack the demo vocals as well.
5. I send the vocalist .wav files with the following stems: lead vocals only; harmony vocals only (if any); and a karaoke track with the full arrangement minus vocals. In addition, I'll send an Mp3 with the latest mix. I'll also let him know the tempo so that he can program it into his DAW.
6. The vocalist will send me a rough vocal take, or sometime a rough mix with his vocal. I'll make suggestions. Then he will record another take. You'll need to work out with your singer how he wants to deliver the vocal tracks. My preference is for the singer to send three or four takes, unprocessed (no EQ or effects, and only the minimal compression that he needs in his input chain, if any. Three or four takes is usually enough for me to comp together a killer finished take, as well as a double if needed. I prefer to do the comping. I can understand why some singers prefer to do it themselves, so you're going to have to work that out.
7. I comp together the lead vocal track and, sometimes, a double. I do any editing or pitch correction that is necessary (usually quite minimal). I'll also edit or retrack the harmony vocal tracks to blend them with the lead vocal tracks. Then I produce a mix. Usually I'll post a working mix on the Mp3 Clinic here at HR.com where the regulars are very good at pointing out problems (in return, I reciprocate by doing the same for them).
Anyway, that's it. Within the next several weeks, I'll be releasing an album of my songs called "Island Hopping" with River/Men, a collaboration with ibleedburgundy (drums) and easlern (vocals) on the forum here. Ibleedburgundy and I have begun recording a second collaboration, still in its early stages, with Jimistone from the forum. More on that in due course.
Hope that helped.