This guide is for saddle fixing on the fast n' cheap. If you want to get fancy about it, get yourself a rivet gun and take the saddle leather off the carrier. We'll cover that technique some other time.
Things you will need:
Saddle. Proofide. Toothbrush. Bucket of water. Toe straps or twine. Heavy things.
I got this B-72 saddle for $25:
As you can see, it has lost its shape and is cracking badly. Looks a little like a toad's head too.
First thing to do is back off the tensioning pin entirely. You don't want any tension while re-blocking the saddle's shape.
Next, use straps to block the saddle into the desired twin parabolic curve. I used a bubble tie on the front as an experiment in the picture, but don't do that on your saddle – it leaves dents in the leather. The toe strap will leave a dent too, but it will buff out after a hundred miles of riding or so.
Another method is to drill holes in the skirts (the side leather that hangs down) and use twine to block the saddle. That will provide a more even curve, but takes more time and effort too. Time and effort is the nemesis of this guide.
With the rough shape blocked out, submerge the saddle in water for about a half hour. If there's dirt on the saddle, this is a nice time to clean it using mild dish soap.
Please note: never submerge a saddle that doesn't need re-shaping. You can ruin it. The leather will be very fragile while wet, so be careful not to stretch it too much.
Take the toe straps off while the saddle is drying, and use heavy objects to hold the saddle in shape. Fancy people will cut a mold out of styrofoam and fill the saddle with wadded newspaper. I put the saddle face-down with a 20 lbs weight on top so that the top doesn't bubble up while drying. On a different saddle I forgot to do that and it took months of riding (by my roommate) to get the top flat again. The saddle will take between one and two days to dry. Check it often to make sure it is keeping the shape you want. When it is completely dry, the skirts will flair out a bit, so tuck them in a little extra with the weights.
Here is the saddle completely dry. Note, I haven't applied any Proofide or leather conditioner to it.
A note on saddle conditioner: I used to be a shoe shiner all through high-school and my first year of college, so I know a thing or two about leather conditioners. In my experience, Brooks Proofide is the best thing for a saddle. I've used Saddle Soap and various Kiwi products, but nothing penetrates like Proofide, or lasts as long. This isn't an ad, just an observation.
The conditioning process should take between several days to several months. I suggest you begin applying leather conditioner while the saddle is still a little damp, and keep applying it every 24 hours for several days. Do not ride the saddle during the initial penetration treatments. Proper method for applying conditioner is to use a toothbrush to work the goo into the whole saddle, including the cracks and crannies, then polish with a rag. Make sure to apply plenty to the underside too.
After a few days of conditioning, lay off completely and set the saddle somewhere warm and dry, like a furnace room or a desert. The saddle will be very weak while it's re-hydrating. You should be regularly polishing the top of the saddle with a dry rag. Once it stops leaving residue on the rag you can re-tension the saddle. This takes two or three weeks. When you re-apply tension, do it slowly: give it one full turn then let it sit a day, polish with a dry rag, and give it another turn tomorrow and repeat. If you notice new cracks emerging, rub in a little more Proofide and wait a couple days before adding more tension.
Here is the almost-finished saddle, after several days of Proofide-ing and polishing. Note the cracks are still there, but they're the same color as the rest of the saddle. Over months of riding and polishing, they'll start to blend into the saddle, smooth out, and look like healed-over scars. Way cool.
The final step is to ride the saddle a lot. That will help it find its final shape, and it will also polish the leather into a nice shine. Wear dark pants for the first hundred miles. The saddle will rub off on your pants and butt. Might not be a good look for white denim. Apply a very light coating of saddle conditioner only if the leather starts to fray or if it gets wet from rain.
Hey, if you like this post, I have a whole series on HOW TO BUILD A BIKE FRAME you might enjoy.