Here are very simple instructions since there's no welting. ;o)

Here is a link that might be useful: window seat

Hi labradoodlemom! I've made cushions for window seats, bolster cushions for day beds and all kind of seat cushions throughout the years. I sew a lot, and upholster my own furniture, and I'm always thinking up some new sewing project or decorating scheme that needs a cushion. I've put in a link to the last place I ordered from, and I was very satisfied with the cushion, and their pricing. You'll find, once you start checking out different sites, that the foam can usually be the most expensive part of your project. It's very rewarding, though, to do it yourself and have it look just the way you wanted it to.

Don't forget to order batting to go around your cushion, too. I usually use a spray adhesive to attach the batting to the foam before making the cover. For a window seat, I'd suggest at least a 4' or more cushion, with good density - you want something that will be comfortable and supportive to sit on. I'd also recommend that you plan on putting a zipper on one end of your cover, so you can take it off easily to launder it as needed.

I do put welting on all of my cushions, but if you're looking for something without welting, I guess the site that patty cakes suggested would work. If you want a link to another site that has more detailed instructions for cushions with welting, just post again and I'll send you another link.

Here is a link that might be useful: upholstery foam

I'm almost finished sewing covers for window seat cushions. I had planned to buy new foam (approx. 4"x26"x77") but after discovering it would cost around $200 I decided to use the foam I already had and wrap it with a thick layer of batting to give it a little more thickness. I found a couple books at the library that had good illustrations and directions on sewing them. I'm a novice seamstress, but I'm really happy with how they are coming out. The books I used are "So Simple Upholstery" and "Cushions & Covers" (by Readers digest). It took both books for me to get clear instructions on sewing in a zipper, etc. One trick I learned on line but have not had the chance to test yet is to put your foam insert in a garbage bag to make it easier to slip into the cover. Then just cut it off and pull out once it is in place. I have struggled getting cushions back in slipcovers after washing in the past so hope this tip helps.
Good luck, it is very satisfying to do it yourself and when I got an estimate for someone to sew them for me it was $300 to sew 2 covers 26"x 38" (with welts)so it's a big money saver as well!

I have always bought cushion foam locally.

Go to a good sewing/craft store near you and ask for high density foam.

I honestly don't think there's a huge difference in the foam, as long as it's high density. And it you're not paying for shipping, the cost will be a lot less. They can probably cut it to size for you.

They may even make covers to order.

Dacron wrap is great stuff. I have also used old blankets and feather duvets just because I had them lying around.

If your sewing skills are not up to putting in a zipper, it's OK to just baste the seam down the back the LONG way, then rip it out when you need to wash the cover. Then put the cover back on DAMP and it stretches itself out. No ironing.

Good luck!

Natsis, that is an excellent tip about using the garbage bag to help insert the foam and dacron cushion into the fabric cover. I've done this many times, and it makes the job so much easier, especially with longer bench-style cushions. I'm glad that you mentioned it!

Would any high-density foam work for outdoor (sheltered porch) cushions or do I need to get "outdoor" foam that doesn't hold water? I checked out the link nanny2a gave, the outdoor foam is expensive. Overstock has foam mattress toppers for $100, a Twin XL would be more than enough for what I need.

I think that might be a perfect solution for the OP's window seats since they're indoors, unless she can find foam locally for less.

nanny2a & others - I'm also interested in making window seat covers & would love if any of you could post pictures of some you've made.

I haven't sewed for a long time, and never made anything with welting. Is it something you would only recommend to someone who is quite skilled? How does it compare to putting in a zipper, for example?

amck,
I can't post photos right now but I just finished my box cushion covers yesterday. They have both welting and a zipper. Neither was very difficult. The welting is very simple just wrapping a cord with a length of fabric and sewing up close to it with a zipper foot. I thought the whole project was fairly simple. I was sort of surprised by how well they fit when finished. I impressed myself! I would not consider myself "quite skilled". I just took my time. The hardest part was struggling with my very old sewing machine that kept wanting to jam up.

Here's just a few that I have in my home. All the other cushions I've made have been for other people, and I forgot to take photos.

Window seat cushion with contrasting welting.
{{!gwi}}

Bench cushion with welting.
{{!gwi}}

No welting on this one, just a simple cushion.

Actually, Overstock has one that's even less than $100 - any size. Would this work for indoor cushion for labradoodle? Would it work for outdoor cushion for me?

Here is a link that might be useful: Memory foam topper at Overstock