fit the seal to the body, then put the glass in, using the string?
No
get the entire glass and seal assembly wedged back in there at one time,
Yes
the glass should be inserted into the seal, and inserted at the same time. You'll want the cord to lay across the top, around the whole seal so it crosses by about 8" or so. Have someone outside to help, and they will hold the whole unit and push as you pull the string out. I'd recommend starting by having the inner seal lip over the seam as much as you can at first. I'm suggesting you start at the bottom first because the weight of the whole unit will help to hold it in. Start closer to a corner, try to get a corner done by having the outside person push on the glass as you pull the cord and pry the rubber up over the window corner. Once the corner is done, work your way all along the bottom, person pushing with you prying the seal up over the bottom lip. Same in the second corner. So by the time you have the two bottom corners done, the person outside is just there to push and the window shouldn't be in danger of falling out.
Prior to starting, make sure the seal is evenly set on the glass. It may stretch or bunch up as you go, try to not let this happen.
If it does, don't be afraid to push the whole thing out and start again.
You will want a full spray bottle of glass cleaner and use it quite liberally to spray the seal and the steel window frame as you go. It is used as a lubricant and is very effective, use lots, it just wipes off - no mess.
When you have it done, look at the corners and make sure the seal is evenly spaced all around. If not, it could leak and it will be hard to get the lock seal and chrome strip back in.
Hope this helps. J