The hatch glass is a bit tricky because the end opposite of your pull wants to push out of the hole. We've always done it with the hatch mounted and you really need some help keeping the glass from moving too far opposite of the pull. I used to use very thin diameter cord but found it more likely to tear the rubber if you weren't careful and/or well lubed. I switched to soft "clothes line" (cotton or poly) cord and have been doing it that way for years. It's probably 3/16" or 1/4" and I soak it before use. Easy pull and no tearing. We've done more than a dozen this way. I think my son did one solo one time but I never have...
Granny is correct on the constant pressure, pulling the inside rubber outward as you go. We pull a bit of cord, press the glass down while pulling the inside rubber outward to help set the glass as we go. The farther along you get the more the glass will be able to settle so the guy on top needs to keep going back with pressure while not allowing the glass to move out of the hole. We normally start on the top but I don't think that really matters. We also don't lay inside - you can reach the string pull from outside and it's then easier for the pull guy to pinch the interior rubber to the outside helping to set the glass and weatherstrip into final position as you go. Keeping the glass down in the hole is paramount. Slow and smooth is fast in setting windshield and hatch glass.