Waterproof, soften and add life to your boots, saddle, or other leather products by using neatsfoot oil. Neatsfoot oil is an oil taken from the bones of cattle and is an excellent conditioner and weatherproofing agent for leather.
CLEAN YOUR BOOTS FIRST! Skip this step if your boots are new, of course, which is an excellent time to waterproof! You may want to remove the laces if you have lace-up boots and just get them out of your way. Wipe or even wash off the excess dirt, oil and grime with rags and/or a soft brush. Then use saddle soap (recommended) or another leather cleaner. Using one rag, scrub the leather down until you work up a lather. (Doing this works additional dirt and impurities out of the leather itself.) Then with a second, clean rag, wipe the saddle soap from the boots. It's best to leave the leather overnight or at least a few hours before moving on to Step 2.
Using a fresh rag, apply neatsfoot oil to boots. It is not necessary to try to work the oil into a lather as you did the soap. Just apply generously, taking care to work the oil into every nook and cranny, before gently dabbing excess from the leather. Don't wipe off the neatsfoot oil!
Let boots stand overnight before wearing. You've just extended the life of your boots by cleaning, conditioning, waterproofing and preserving the leather.
I use old, worn-out socks to apply and wipe saddle soap and neatsfoot oil. This way I can just toss them out after use and not worrying about cleaning or re-using them. You can purchase saddle soap and neatsfoot oil at almost any feed store or ranch supply and some saddle or boot repair shops, along with some hardware stores. Anything is available online, of course. Google it and get it coming.
Don't use hydraulic or motor oil to waterproof leather. They may work to resist water, but they also shorten the life of the leather and the chemicals in these oils are harmful to humans as well. Nor will your feet be fireproof!