For a light cleaning: 

Use a moderately damp brush or cloth to wipe away any dust and dirt. Use an old toothbrush to thoroughly clean the areas around the leather upper,; sole, and creases where caked-on dirt and gunk may have gathered, causing the material to stain and scuff.

For a deeper clean, the process is a little more intense:

1. After removing dirt and debris with a soft cloth, add a leather cleaning product 

Saddle soap cleans and moisturizes leather so it will last longer, it can also be used on materials other than leather. In a circular motion, add saddle soap with a wet brush or cloth. Work up a foam on the surface as you add it to the entire boot. Rinse the boot until clean. Use a clean cloth to wipe away the excess lather from the leather boot.

2. Let your boot air-dry without heat

3. Condition the leather with oil

Allow your clean work boots to dry before this next step but don’t want too long or else the leather will dry out from the cleaning. Work the conditioner into the leather all over the boot. Conditioning helps moisturize the leather to prevent drying and cracking. If you want to give your boots a shine, now is the time to do it. As you use a clean cloth to remove the excess conditioner, buff the boots with long circular strokes and give them a nice shine. Note that if the booth is suede or nubuck, the oil will darken the leather but it will nourish and help keep it from drying out