You can easily wash your Tau product by hand and then hang it up to dry. Please do not put the product in the dryer.
Biothane is very easy to clean. You can simply wipe your product with a wet wipe.
The fat leather does not need a lot of care, it is very robust.
If your oiled leather product gets wet frequently, you can treat it with oiled leather care oil, but you don't have to because the leather will become softer again over time. For the maintenance oil treatment, the product should be dry and clean. The oil should soak in for an hour or more and the color of your product will be a little darker after the treatment.
Note on brass
Brass tarnishes a little over time, that's natural.