Ok, going to reply a bit more completely now - I was on my cell phone and you know what it is like typing on one of those!
So, you built the frame, right? Do you have it all together or just the base 4 frame pieces? I'd recommend getting it in place before you put the panels in. Moving it risks racking the frame so I'd do it before the panels so that you can square it up if it shifts in transit.
As for accessing the back of the greenhouse once it's in place, I found that I could move the whole greenhouse and base with small pieces of PVC under the edges of the frame, kind of the way the Easter Islanders were supposed to have moved their statues (I watch too much educational TV), but if that isn't an option for you, do the back wall first so that you can reach around each panel from the inside. If you are planning on using caulk instead of the clips, you could do this easily. If you are planning to use the clips, it might be tricky to get the last ones in. You will have to see what is easiest in your situation.
I'm going into my third winter with my little greenhouse. I found my pool cover so I will reinstall it this winter. We raised the greenhouse an additional 12 inches to give me a bit more headroom, but I haven't yet rebuilt the door. That means I still have a bit of a step up to get into the greenhouse.
I haven't yet replaced the door - hopefully I'll have time this winter to build a nice one out of oak to match the fan frame (pictured in an earlier post). I'll be cutting out the 2x12" board at the door and reinforcing the sides before putting the doorframe and door in place.
I tried something new this summer. Since the greenhouse was essentially empty over the summer, I was hoping to reduce the UV degradation of the polycarbonate by covering it. The dimensions of this greenhouse mean that a 4x8' sheet of foil backed insulating foam cover both the sides and roof planes of the greenhouse! I bought two of them, taped them together at the peak (leaving a bit of space for them to fold like a tent) and dropped it onto my roof! It covered really well and the duct tape I used to form the hinge between the pieces held the front, back and sides of the little foam tent onto the roof. We had some pretty wild weather and it never came off, though by the end of summer, the UV had taken its toll on the tape's adhesive. Made it easy to take off at least. Both foam panels fit inside the greenhouse against the north wall and will hopefully provide some insulation this winter.
The other addition I made was to the floor. I tried sealing it, but really can't get to the bottom of these boards now that I've raised the greenhouse. The boards are pressure treated so hopefully, they will not rot away too quickly. This past winter we had some wicked winds and the insulation I put under the platform obviously has fallen away from the boards so that wind came right up through my nice floor! This winter I will be better prepared. I went to the feed store and bought a couple of stall mats - those heavy ones that are made of recycled tires - and put those on the floor. With a tiny bit of trimming of the length, two of them cover the floor quite nicely!
The fit was tight when I installed it, but the mats shrink a bit when it gets cold. I might have to do something about the gap between the two mats (duct tape?) to keep any drafts out as the temperature drops, but the fit in the corners is tight enough I don't think we'll have any leaks!
I've just put the porch plants into the greenhouse and started using the heater at night. This is the time of year when you have to alternate between a fan in the daytime and heater at night, LOL! But at least I can keep a bit of my green, green! Happy gardening and hope this post helps.