Shoes and clothes made from leather needs to be anointed frequently to stay soft. The anointment will also provide protection against moisture and water.
Here's a short how-to-recipe:
Put the fish liver oil (tran) in the pot. It does not matter if the liver oil is old and smelly. Use liver oil from cod, herring, catfish or any other fat fish.
Add the tallow.
Heat it til it boils and until the foam is gone - the tallow will merge with the liver oil.
Add a little tar (Tjære).
(Optional: add a little beeswax).
Let it cool down. Do not apply while hot, this may damage the leather.
Look at that lovely color!
If you can't get you hands on tar, sheep tallow and fish liver oil you can cheat with leather fat from Gold Quality Læderfedt Natur.
Its without silicone or turpentine - made from vaseline, veg. oils and beeswax.