Like with all leather items, you just wash them by hand with dish soap and water. Don't immerse them in water, just use a washcloth. Then wipe them dry and hang them out to dry.
If they're still stinky after that, you can try using a diluted solution of bleach (read the label on the bleach bottle to know how diluted). Apply the diluted bleach, leave for about 5 minutes, and then wipe off with a wet washcloth. You can also try covering them with baking soda overnight and just shake out the powder when done.
In your case, you should probably spray them with diluted bleach even if they're no longer stinky after hand washing them. After a year and a half of exposure to sweat and skin, you probably have a bunch of pathogens on the surface of the pads. The diluted bleach will kill whatever little critters are on there still.
One bit of warning about leather and bleach: Bleach can damage the finish on leather products. Whatever you do, make sure you test it on a small spot first. If you don't see any problems there, you can use it on the whole thing. Pay attention to the dilution amount and the time needed. Too little, and you won't kill germs. Too much, and your leather will be discolored and may become damaged.
Ideally you should wash every piece of clothing (including pads and belt) after every use. It's a pain, but it can prevent all kinds of health problems.
Here's a good video on the subject: