I just noticed this, so maybe I can offer a little help if you haven't done it already.
All you need to remove the seat cover is a screwdriver, some sockets and some hog ring pliers. They're essential, and you can't get away without them, since you'll need them for reinstalling the new hog rings.
All you have to do is remove the seat, undo all plastic parts and seat base/sliders, and flip the seat upside down. You will see a set of rings holding the upholstery to the seat frame. These are the hog rings. Take the hog ring pliers (you can get them from Sears, Snap-on, wherever, just make sure they have longer contoured handles, or your handles will get tired FAST) and grab onto the ring. The objective here is to open the ring back up and spin it out of it's place. You can cut them and just take the pieces off, but it's harder, takes more time, and more likely to loose pieces in the seat frame. So, if the ring's ends aren't overlapped (this depends on how tight they are and the size they used), then put the plier ends 90 degrees from the opening, compress slightly, and pull. This will be enough to have the frame force the opening of the ring so it can be removed easily. If they are tighter and overlapping, just grab on a ring end, pull and bend it up, then grab the ring and twist it out. They're tricky, but you'll get the hang of them quickly. If you don't have hog ring pliers, they're a much bigger pain. Once they're off, you can figure out the rest.
The headrest, though, is also a bit tricky. I don't remember how it's retained on the older seats, but on most car the head rest is restrained from coming up all the way by either small circlips, c-clips, or friction ball down in the seat at the end of the shafts. Depending on the type, use either needle nose pliers, an awl/pick, or a flathead screwdriver to compress/remove them. Don't just pull up, as this is a common place that gets damaged in disassembly because they don't bother to look, and it's a pain to fix.
Once that's off the headrest cover generally comes off the same way. Sometimes they're finish sowed on the foam, but I don't remember the Mustangs being like this.
Then you can wash the covers in a regular washer/dryer, I just suggest washing cold and either air drying or not drying too quickly. The heat will shrink the cover, making it a really big pain to reattach.
Personally, at this point, it would be better to just buy some pre-made seat covers from someone like Katzin or one of the various OEM replacement suppliers. The former could do any color you want. It'll cost a bit more, but it will come out much, much nicer.
If not, now that the seats are cleaned, you can dye them any color you want. There are various upholstery supply stores online that will have what you need, just call them up and explain what you want and they'll tell you. They should also supply you with the color-treating sprays that will seal the dye into the fabric and ensure that it will not fade or blotch over time. Skip that step, and that dark red will look like a murder scene in a couple months. For this application, most all dyes and sealants will work fine. Also, buy a good bit of spray-on contact cement- it's a crucial step most people skip, and why most "home" jobs are wrinkly and loose. However much they suggest you buy, double it, because you WILL mess it up. Don't forget a box of hog rings, too. Buy the goods from an upholstery shop, either locally or online, and DO NOT go and buy crap from Autozone or Schucks or something. If you do, it WILL turn out like crap.
After dying it, by the way, I don't suggest you wash it in a washer. Dye, seal, seal, spray with good interior fabric cleaner/degreaser, then seal again. The washer is too harsh and will fade the fabric instantly. Dying fabric is not like dying jeans.
Once that's done, make sure the foam bases are good, clean, and dry, and spray with a thin layer of contact cement. It should be a very thin, spread out mist; it shouldn't have visible depth and look like spider webs. If it looks like a web, then you've either got too little pressure (globby), too much pressure (stringy) or not mixed enough. This is assuming you use a can. If you have access to compressed air, that's better, but requires a lot more equipment so don't worry about it. Let the cement cure for about a minute to 2 minutes until it's drier, but still tacky. Wait too little, and you'll be a sticky mess. Too long, and it won't stick. Then, beginning reattaching the fabric, INSIDE OUT. It's impossible to try to do otherwise. Start on the end and slowly press more of the cover on, making sure you press from the inside out. More than likely, you'll have to do this twice to get it attached without wrinkles or bubbles. Try not too take TOO long, or the cement will dry entirely, but make sure you don't rush yourself. Go about an inch at a time, from inside to outside, top to bottom. Once it's satisfactorily attach, immediately reattach hog rings to the bottom of the seat bases through the fabric. I usually try to add a couple to the OEM ones just for extra security against wrinkles or sliding. Reattach the headrest the same way, etc, etc, and let it sit for at least 30 minutes. Then, but it back together and into the car!
If you have any question, PM me, or if you want help, let me know and I can probably help you out with it. I REALLY am not a big fan of dying fabric generally, except for carpet, so I highly suggest just buying new covers, but this method should work if you don't want to spend the cash.
It's going to take you about 2 days of work or so.
