Michael, when you read a whole set of directions for a sewing project like the G4, it is easy to be overwhelmed, confused, and discouraged. However, when you start to sew or cut out the fabric pieces and you follow the directions one small step at a time, the following step will suddenly become quite clear and obvious.
The most important tool to have handy is the seam ripper. Even the experts make mistakes. The seam ripper helps you remove the semas you have sewn by mistake. Also, when you are sewing along, and all looks good on the top, stop and look underneath the seam. Make sure the stitching looks as it should. I mention this because sometimes the underneath is all balled up or too loose. This happens if the thread has been threaded wrong into the machine or if there is a bobbin problem or a tension issue. Nine times out of ten all will be fine, but check just to make sure.
With the G4 or any pack, the tricky part is the shoulder straps. They need to be placed carefully paying attention to how far apart they need to be. They also need to be strong where they are sewn.
Anyway, my main advice is to just start in, sew one step at a time. The real fun comes when you are done!
PS I am glad Lance mentioned pinning. It is often tempting not to pin, because it saves time and is easier not to. Using many pins is key to sewing a curve, but even on a straight edge, if you do not pin, it is easy to have the edges uneven. When you get down to the end of the row, you suddenly find the bottom is an inch or two longer than the top. Whoops! Sometimes it might not matter, say on a tarp, but you do not want to end up with a lopsided pack.