Once I had the front stitched, I picked out a vintage sheet for the back that I had on hand and pinned the top to the bottom. I hope you're not looking for a tutorial, because I made this up as I went along! I picked up some giant red rickrack at Joanne's for a trim between the sheets. I pinned it the best I could, but as I sewed, the sheets moved along and my sides did not match anymore--groan! So, I would re-pin the other sides and hope for the best. Which brings me to another reason a duvet cover was a better choice for me: they can be rumplier and not perfect and it's OK. Whew!
Once I had them all toghether, I sewed some ribbon on the inside corners of the duvet and the corners of the down comforter I bought at Ikea. That way I can tie the comforter to the duvet and it won't shift around annoyingly. When I say this thing is not perfect, it isn't false modesty! Here you can see that on one corner the top sheet is about an inch longer than the bottom sheet. But when it's all tied up you can't tell. :0)
Aaaaand here is a hole I accidentally snipped in the bottom sheet about 2 seconds after I finished the thing. Thankfully it's right at the bottom and when I stitch it up you won't be able to tell. Oy.
Instead of risking more uneven seams, I just made ties for the bottom out of the extra fabric from the top sheet. It'll be easy to get the comforter in and out this way, too.
And here it is! I'm so glad I splurged on the rick rack. I think it really makes it. Can't wait to show you what it looks like on the Big Girl bed! Life has been busy, but we're making good progress on the room, and my grand to-do-before-the-baby-comes list. :0)