If the seal is not damaged, you should not need the silicone. Make sure all mating surfaces are clean. If the rubber is damaged, torn, cracked, etc. then you will probably need some kind of sealer.
The hatch glass is easy to do single-handed, because you can lay the glass on the open hatch. Use sash cord as above, or something around 3/16" thick rope. Install the seal on the glass, lay the glass on the hatch, and put the rope into the groove of the seal so that the two ends cross each other at a corner. Now carefully pull on the rope to feed the inner lip of the seal over the lip of the hatch. Once you have the whole thing in place it makes sense. You can zip in a rear hatch glass in about 30 seconds.
Note that my method is the reverse of Old Man's: put the glass into the seal first, and use the rope to pull the seal onto the hatch. The groove for the pinch weld (or equivalent on a fiberglass hatch) is looser than that for the glass, and is made for this procedure. It is also designed specifically to be done from outside in. The outside flange on the seal is tighter and deeper than the inside, making it very difficult to do the same thing from inside out.