I'm no expert in any way, but I've learned so far that if I don't soak the leather piece completely and dry it slowly without any aid, it will shrink in some places more than in others during dyeing.
The first time I put completed sheath in melted beeswax after only wetting the bend, the sheath shrunk close to 25% (I know, I know... wax was too hot, I didn't work on the inside enough).
Since then I did it with 3-4 other sheaths and shrinkage would not exceed 2-3%, and for those leather has been water soaked and slowly dried 2-3 times before I started working on them.