It helps if you know how the leather was originally waterproofed i.e. silicone, etc., and stay with the original method. Otherwise, I agree with CC that Sno Seal, which is a dissolved wax treatment, is the best choice. I do not like any of the oil or grease treatments (neatsfoot, mink oil, etc.), as they cause the leather to stretch. That has been my experience in 50+ years of using leather boots. I also prefer leather laces, but those are becoming more rare every day. Taslan also works fine, if you can get good quality laces.