sewing this THING that is now folded in half with the seams trimmed to exactly half inch, and hopefully notched and recognizable as labelled can be done in several ways:
One: from the position it's in, not partly or wholly disassembled at the table, but only unpinned as sewing.
Two: methodically unpinned and repinned to be in an open unfolded position. At no time have you unpinned more than one seam before repinning in an unfolded position.
Three : Unpin and cord the outside body pieces before unpinning any of the inside body pieces. I will tackle this first.
Four: and I don't suggest this, unpinning wholly, so as to overlock before hand. Yes, I do this. Yes, even I get lost. (For the sake of overlocking in a single layer, I suggest you do this only after mastering method one. )
All the outside body pieces I have ever pin fit ( say 15,000 ) have yielded a flat puzzle piece, free of folds or darts. If this is not true for you, and you do not have a very unusual piece of furniture, I want to to recheck to see that the seam location you chose is as far to the outside of the arm or back roll as possible.
You want these pieces to be flat. Why ? When I say you can cord them easily, this should be true, that you can sew~feed cord onto the single layer, with skill of course, and not distort the good fit that you cut. Outside body pieces that get cord at this time are the outside back, arm front panel if present, the outside arm, and if present, the wing. If you have a separate panel that defines your deck front, do not cord this at this time.
Though this looks awkward in explanation, to keep the writing brief, I will now tell you how to open pin the double pin fitting ( option two ). You would procede to open pin the inside body pieces if you first corded the outside body pieces in option three.
Let me say this again: if you are doing option three, we are going to unpin and repin the remaining uncorded slipcover pieces. If you are following option two, you are going to do it the same way.
Notches are critical here, so if you see a need to add a few, or write in instructions or take notes now, do so. Unpin one line of pins only. You will see four layers now hanging free. Take the two closest to you ( facing you in their entirety ) and repin at the notches you have made. If you find none, make them now.
Once you have repinned the two layers closest to you, turn the pin fitting over to the side that has no pins showing. You will see two edges hanging free needing pins replaced, and having the same notches you just connected. Hint: you can use many straight pins or fewer safety pins. You should place them where they were before, at the seam allowance you decided on.
Once secure and still recognizable as a seam, you may move on to another line of pins. I don't see why you would not be able to do this in any order. The long straight seams are the easiest to recognize, and the tight or short seams the hardest to get at, and can be left for last for convenience sake.
At this point I am going to assume you are ready to hear option one, as the explanation for one is the same as the explanation for picking up where I have left off for two and three. It looks awkward and makes you jump around, but it makes the point that you need not have done the extra steps for two and three, one is the shortest route.
Option one: if you were sitting at the machine, cord prepared, ready to sew for several hours without distraction, you could unpin and cord all in one short sequence, avoiding some possible confusion and extra steps. Not to contradict myself, but if you tried two and three first you'd be guessing less at this point. You'd have a better visual picture in your mind of what the task is, and be ready to do it more efficiently.
Looking at a double on half folded pin fitting is difficult, if you are trying to see what to sew. You must eliminate the few things that you can sew without cord first, the tuck ins. I hope you have notched mid seam on all the tuck seams you have cut. At minimum you have inside arm and deck to join, perhaps inside back and deck, and then certainly the inside arm where it meets the inside back. Sewing wing, if present, at it's tuck in, is also appropriate at this time.
It is proper to insert in the explantion here that any uncorded seam can be started and stopped at any point and can be sewn in more than one segment. You need not sew from A to Z. You may sew, and may benefit from sewing, M to S, F to M and then leave S to Z and A to F open for now. Those letters are aribrary and not assigned meaning in any previous paragraphs. Just to say, sew what you can know now, leave the rest for later when it becomes more obvious.
I am going to leave off here for today, with you closing your inside seams where no cord is required, and leave you with the task of examining and memorizing the THING you are looking at.
Visual memory is a skill, and it needs to be developed and trusted.
When you close those inside seams, I am suggesting you do so either from the front ( end nearest to the outside body pieces ) or from the centers of those seams toward either end. Center out for now is adequate, front to inside most~end is simply more effcient and will be used when your understanding increases.
Please know that all this torture is very profitable. In the shops which employ piece rate seamstresses assembling slipcovers, a chair can be completely sewn in two hours or less. The experience of the sewer can yield a quality cover using this procedure, it does not cut corners or result in poor product.
I have not said anything about where the cords GO, or where they stop ans start. I will generalize about this next time, but you need to have decided upon some aesthetic points prior to cutting, and your cord order, stop and start points will be a reflection of those seam location choices.