First off, I'm not an expert and I don't play one on TV...
That said, this is how I'd attack it:
Since it appears that there is nothing from the C pillar back, and no reference points to work from; it's all going to be mock-up to start with. Get lots of clamps, clecos, sheet metal screws, duct tape... whatever you can lay your hands on.
Clamp the dutchman roughly in place. Get your trunk floor and dropoffs clamped on, and your tail panel.
Then trial fit one quarter along with the trunk drip channels, adjusting your clamps on the previous panels as you go- start from the rear door jamb and work your way rearward, adjusting as you go. It is beneficial to temporarily hang the door (and even the front fenders) so you can keep your body lines in alignment. Now repeat for the other side.
Keep in mind that NOTHING is permanently attached at this point- more adjustments will need to be made.
Now trial fit your trunk lid, paying close attention to the gaps- finesse the panels until you're satisfied with the fit. This is a good time to lift your bumper in place and check the fit, and adjust if necessary.
This is going to take a while...
Once all the fits are where you want them, start marking every panel, every joint and seam, every mounting point, with a china pencil (don't use a Sharpie!) and painter's tape so everything can be reassembled EXACTLY as it is now. Lots and lots of marks.
Now you can remove your quarters and begin tacking all your replacement panels in place- start at your existing hardpoints (frame, window/body structure; trunk floor and dutchman) and work your way outwards- and keep refitting the other panels to make sure your reference marks haven't changed or moved!
Once you're ready to hang the quarters, quadruple-check your alignment (you only get one shot at this) and your trunk/bumper gaps. Tack them in a few places; door jamb, rocker, C pillar, tail panel. Check your alignment AGAIN. Make sure it's clamped tightly everywhere, and finish attaching.
This is just a rough outline, but I think you get the idea.
I'm sure others may have other ideas to add or change, but the main idea is to put things where they belong on YOUR car- keep in mind that the replacement panels are good, but they're not perfect, and each car is a tad different. You most likely will have to do some massaging on those nice new panels.
Hang in there, stock up on refreshments, and don't rush yourself.
Good luck and keep us posted!
