All right…on to sizing your pillow cover and how to close it up.
There are many different ways you can close your pillow cover. The most straightforward is to sew up three sides of the pillow, stuff it with filling or a form and then handsew the opening closed with a slip stitch or ladder stitch. You can also close it by machine, but you will have to sew very close to the edge and the stitching will be visible.
An envelope closure is similar to what you see on the back of most bed pillow shams. There are two pieces of fabric that overlap about 4" to 6" when closed, but easily separate to insert or remove the pillow from the cover. To make an envelope closure, you would cut two separate rectangles that would be the same length as the front square and about 2/3 the width. Using the 18" pillow as an example, the front would be 18" x 18" and the two back pieces would be 18" x 12". You sew a hem along one of the 18" sides of each of the back pieces and pin them to the front so that they overlap in the middle. An envelope enclosure can be easily adapted to using buttons or ties where the back pieces overlap, too. You could put the envelope portion on the front of your pillow to showcase some pretty vintage buttons or special ribbons and trims.
My favorite way to close throw cushions, and also what you would use for a window seat or bench cushion, is to insert a lapped zipper. I know it sounds intimidating, but it really is very easy. You need a zipper that is 2" shorter than the width of your pillow. So, for an 18" pillow, use a 16" zipper. The only other things you need are a standard zipper foot and a marking tool. My zipper tutorial shows you how you insert the zipper sewing four straight lines. That's it. I really encourage you to give it a try. I'll be putting zippers in almost all the pillows I sew next week so I'll show you how easy it really is.
You can also use an invisible zipper in the side or bottom seam of your pillow cover. To do this, you need a piping foot or an invisible zipper foot that is often sold for a few dollars in the fabric store right next to the zipper display. I did a google search for a good tutorial and there are many solid ones out there, even some video ones. If you think you might want to go that route, I highly suggest doing a search and then watching or reading about it first. The invisible zippers are pretty easy to install once you understand what you need to do. Also, the bonus of an invisible zipper being in the seam of your pillow is that the finished cushion is completely reversible and you really don't see the zipper at all (yes, invisible!!!).
I think that rounds up the most commonly used closures, but if I missed something, please let me know in the comments. As far as fit goes, it's really a matter of personal preference. You probably didn't want to hear that it is so subjective, but it is.
If you are using pillow forms, you first need to decide how stuffed you'd like the pillow to be. I like a well-stuffed pillow and consequently cut my fabric to the exact size of the pillow form. Let's say I'm making an 18" square pillow – I'll cut the front to 18" square and the finished size of the back would also end up being 18" closed. I sew the squares together with a 1/2" seam allowance which gives me a 17" finished cover. The 18" insert will fit without a problem and it's a nice fluffly pillow. If you don't like your pillows well-stuffed, then cut your fabric 1" larger than the insert's measurements. If you like a loose fit, you can cut the fabric even larger. It's all up to you. If you sew the cover, insert the form and it isn't fitting the way you'd like, try a smaller or larger pillow form. That'll probably do the trick. Remember that down compresses more easily than poly-fil so often requires a tighter cover.
If you are stuffing your pillow with some kind of filling, you can sew whatever size you desire and then stuff it to your desired firmness. Really pack it in the cover, getting all the corners if you like firm pillows. Use less stuffing for a squishier cushion.
If you are using foam for a seat cushion or a window seat, you want an exact fit. Take measurements or make a paper template of the shape and cut your fabric 1/2" bigger all the way around. Sew it with a 3/8" seam allowance and insert the foam. If the cover is too big, you can sew another 1/8" seam just inside the first one to get a closer fit.
I'll hit on piping with my first pillow(s) and talk a little bit about other trims then too. A big link list of pillow tutorials coming tomorrow! And then Sunday, it starts! I think I'm ready. You?