Have you ever bought an old camera -- especially one with pebbled leather? Did you notice that the low spots in the leather, between the pebbles, was white or grey? That's grime, dead skin and dried sweat from the previous owner. You need to get rid of that. Vintage leather is good, but the vintage crud on it is not. I use a soft toothbrush with some saddle soap to get rid of it and restore its original solid black color. It will get rid of nearly all of the crud and dirt, but it also dries out the leather by removing many of the natural oils along with it.
After using the saddle soap, I use mink oil to replace the natural oils in the leather and keep it from drying and cracking. After using the mink oil, I have to do a lot of wiping to get rid of the excess mink oil, but it is a necessary part of leather care. I don't use most of the so-called "leather conditioners," and other products that soften leather because many work by breaking down the leather fibers to make the leather softer, and I want my leather to last a very long time.
Now I have to do something to protect the leather. Clear shoe polish goes on next. I use colored polish only if the leather is scuffed or stained. I have my favorites, but I'm not going to get into that. I rub it in well, give it a few minutes to slightly harden and wipe off the excess. Now I start buffing. The longer you buff, the shinier it gets. Start with a brush and finish with a soft piece of cotton cloth. If you want a seriously shiny surface, go over it next with a shine sponge (silicone).
If the leather on your camera is snagged in places, you can take a toothpick, dip just the point in bookbinder's glue (dries pliant, like silicone rubber) and glue the snags down.
If your camera's leather is missing or ruined beyond the possibility of restoration (holes worn in it, deep cracks, red rot, and etcetera), You're probably going to have to replace it. If you're pretty good with a pair of scissors, using a hole punch, measuring and making templates, you can just buy a sheet of leather or leatherette and cut your own. Use some judgment here; leatherette doesn't rot, so if you live in a swamp, a jungle or near the ocean, leatherette is probably going to be a better choice than leather, even if the leather looks a little better. If you are not particularly handy, then there are companies that sell precut kits with self-sticking leather and leatherette, like Aki-Asahi Camera Coverings and Camera Leather. aki-asahi.com/store/ www.cameraleather.com/
If you can handle it yourself, all you need is a sheet of skiver leather (leather that is very thin -- 1mm or less -- about the thickness of a pasteboard playing card) and a few tools. The toughest leathers are going to be goat, stingray and kangaroo, so those are the best choices. Be aware, though, that the surface of stingray skin is all tiny bone knobs and it is VERY difficult to cut accurately. If you're cutting your own, or if you're covering a camera with rounded or three dimensional surfaces (like the lens doors on Retina or Voigtlander bellows cameras) it's probably better not to get the self-sticking kind of leather. Self sticking leather is stiff, won't stretch and you may wind up with a situation similar to trying to wrap a bowling ball in paper without getting any folds or wrinkles.
The first stage is obviously going to be removing the old leather. It will save some work if you can get it off in one piece (you can use it as a pattern), so make an effort to do so. Sometimes this isn't possible though; a couple of examples: some of the WWII era Zeiss and Voigtlander coverings crumble when you remove them (they're not much more than thickened and molded paint over a piece of gauze) and some FED coverings are made out of something like Bondo (has to be removed with a chemical paint stripper). Anyway, remove the old leather any way you can and then remove the old glue. On lots of older vintage cameras they used shellac as an adhesive and that dissolves in alcohol. Most modern adhesives dissolve in acetone (nail polish remover). You may notice a few green bumps under the leather; those are called "Zeiss bumps," after the cameras that are most infamous for them, but any camera with brass rivets in it can have them. It's what happens when the ends of those brass rivets come into contact with moisture and oxygen -- verdigris builds up on them. Remove that and polish the end of the brass rivet with steel wool to get rid of every trace of verdigris. Put a little thin dot of paint over the end of the rivet so it doesn't come back, and your children and grandchildren won't be dealing with the same problem years from now.
Now for cutting the leather to fit. In order to do this, you are probably going to have to make some templates. On most cameras, you start by measuring the diameter of the lens base (fixed lens cameras) or mounting ring (cameras with interchangeable lenses). Next you'll get a compass and make the same size circle on a sheet of paper. Cut out the circle and put the piece of paper over the lens mount (if your camera has interchangeable lenses). If your camera has a fixed lens, cut the piece of paper in half, through the center of the circle you cut out and butt the cut out circle up against the lens base. Hold it firmly in place and rub the paper with the side of the pencil lead along the edges of the metal flange that surrounds the leather. Locate where any other holes are going to have to go in the same way. Cut it out and check it for fit. If it doesn't fit perfectly, tape slips of paper along the edges until it does.
Now tape the top of the paper template to the back of your sheet of leather, using double sided tape, and very carefully cut around the edges of the template. If you have some hole punches, you can punch out any other holes needed; if you don't, use an X-Acto knife, measure carefully, and just be very careful and take your time.
Well, all that's left is to glue the leather down on the camera. A glue stick is all that's needed for gluing down flat panels. If the leather is wrapped around rounded surfaces though, you'll need something stronger; traditionally contact cement is used for that.