There are two primary types of leather tanning---oil tanned and vegetable tanned. Which type was used to tan the leather you are working with makes a difference in how you treat the leather. Use oils like mink oil on oil tanned leather and waxes like Snowseal on vegetable tanned leathers. Generally, oil tanned leathers are softer and more supple. Vegetable tanned leathers tend to be stiffer, so using oil on them will tend to make them soft, which depending on their use, may not be a good thing.
If I am using Snow seal, I will heat the container in a water bath so that it melts, then brush it on. I find that it penetrates much better that way. Sometimes I will rub it on with a rag and then put the leather in the microwave for short bursts until it melts in. If using the microwave method, you must make sure there are no metal rivets on the sheath, or metal eyelets or shanks on a boot. Heat destroys leather, so another thing to be careful of is to not let it get too warm.