The chair that these covers go on looks like this. The seat itself has side gussets but the seat back gets its contoured shape without gussets, just two fabric sections, the back one wrapping around toward the front and cleverly seamed. All four sides of the seat Velcro to the underside of the chair frame and the lower edge of the seat back also Velcros to the chair frame.
Here are the seam lines marked and some key alignment points for assembling the new covers. As is typical of IKEA's ingenuity, two curves in the upper corner are joined offset and this makes for a smoothly tapered and also sleekly curved design feature to the back to the chair. That is another BIG benefit of cutting the cover in half. I still have an intact model to look at when I go to sew the pieces together. It gets tricky where the seat back mates with the seat itself and tucks into that crevice where the two meet, while further contouring around the chair legs.
Then I truly got brave and realized I could mark boldly where the seams were by gliding a Sharpie pen along the seam lines before they were opened up. I could also use the Sharpie to label what the various pieces were. This technique seems obvious now but initially this approach did not occur to me. It probably never would have, had those seams cooperated more in coming apart. I am lucky that those covers were a light solid color so I could write directly on them and read it afterward. Here is half the cover disassembled and on my rotary mat. It was not until I got to this point that I answered the obvious question of just how many pattern pieces there would be. By arranging them on the gridded mat I also got a sense of how much fabric I would need. The mat size is 36" x 24".
The fabric is 54" wide so I may have enough. Perhaps I will run the fabric cross grain. The print does not look like it would appear odd to be run sideways. The tape measure shows a side width total of about 27" so I just might be able to eek out both sides of the seat back and the seat top itself from a single width of fabric. This allows for no shrinkage whatsoever. The fabric is 100% cotton so the covers could never be washed if my DIL wants them to fit again. With a toddler under two, non-washability is not a desirable trait. I am working to avoid cutting it so close if I can. I am blogging about this now so I can have these thoughts circulate in my brain for a bit and so I can procrastinate actually making any cuts.
My backup plan if I cannot fit all these pieces on the yardage I have is to make the side gussets in a different fabric. If I finally found the extra yard and a half after doing that, I would really, really be frustrated.
Here are some of my calculations and plans. I will need about 10 yards of Velcro. I do not know how IKEA sells these covers as cheaply as they do - the lowest price plain white one is $10 and a beige linen-like one is $20. Even disregarding the time invested, home sewing these covers does not make economic sense. But the gauntlet has been thrown down and I am not backing down from the challenge. Truly, IKEA's fabric options really were kind of blah!