Once I'd run out of my flower-headed pins with cutting the pattern pieces from fabric, I started pairing them up and sewing them together. After going back and forth to the computer screen and flipping between the 12 photos I took of the layout, I decided to print them out and puzzle them together. I edited the photos first, removing any colour and increasing the contrast and sharpness, then printed them at about 20 x 25cm, trimmed the margins and puzzled then taped them together as neatly as possible. This is taped to the wall next to my pinning/sewing space, and I mark each seam with a wiggly line as it's sewn. It worked really well; I managed to keep track of what I'd done nicely.
I divided the pieces into piles for each 10 (1-9, 10-19, 20s, 30s and so on) so they'd be easier to find, and that worked well, too. I did find it frustrating when I'd work out a pair to join, then find I hadn't cut one of them, but that'd be solved by buying more pins - which I've been meaning to do for ages and now done - for next time!
Because of the way I drew out the curved shapes, some seams need to be sewn before others - although with some it's less important. This keeps the sewing lines as smooth as possible. I've found that the registration marks are beyond helpful, although I wish I'd put a few more in in places. On the whole, the curves sewed together really nicely. I pin as little as possible - at the registration points, any cross-seams, where there's a big curve, and from the first registration mark back to the beginning of the seam. I leave the end unpinned, and just ease the edges together under the presser-foot as I sew. The main difficulty is getting the ends of the seams smooth at the same time as ensuring that the ends of each seam are completely encased in the next seam. It's all pieced using my usual 1/4in foot.
I pressed with low steam as I went, just to make sure they're lying nice and flat, and the seam allowances stay neat when I sew across them. I was worried that the unmeasured and hence somewhat inaccurate and varying seam allowances (added by hand/eye) might mean the top wouldn't lie flat and square. But that's not been an issue and the few small problem areas are easily pressed into submission with a little steam.
I finished the background late last night. Above is the whole pieced top (apart from the sun; I just need to decide on the fabric) and below is a closer section. It's lying square and is accurate size-wise. Although it doesn't lie entirely flat, I'm confident it will do once quilted.
After the sun (and I also need to add some flame-shaped rays) it will be time to add the planets etc., so tomorrow I'll be painting some fabrics to get the right colours and textures.