From a flyer of the Headwaters Leather Guild - 2006:
(I will add my own personal notes in green)
Always treat your newly bought veg tan leather surface with respect - even air exposure, no moisture, no oil, no fat. Wash your hands before you handle the leather - sweaty fingerprints may only become visible when you add dye or the final finish.
Roll leather up with the grain side on the inside - the other way round will cause the grain side to stretch (slightly) and when you straighten the leather, it might wrinkle.
Rolls of leather can easily be stored in PVC pipe lengths with a diameter of 8" / 20cm or more. Another reason or rolling the leather with grain side in - the nice grain side will not scrape on the pipe edges.
When you start wetting leather for tracing and swivel knife cuts and tooling, make sure to always wet the full surface of the leather. If you do not, the water will rinse chemicals to the edge of the wet part and as it dries, that will show as the edge of a waterstain. (These stains can be treated with excessive washing with a lot of water.)
If I work on a very large piece, I wet the part I am working on with a sponge and then now and again just keep the rest damp with water from a spray bottle.
OIL and WATER do NOT mix.
During the following steps in your project, no oily of fatty substances or containers with finishes in or hands with all of the above on, should be allowed close to your workbench: Cutting, Tracing, Swivel Knife Work, Tooling, Dyeing.
After doing these, you treat the leather with a conditioner / sealer / finish and now you cannot use water on the leather again easily - the leather finish you use should protect the leather against water and against the rougher treatment of construction.
Wet leather and Ferrous Metals Do Not Mix
These metals will stain leather black when it touches even just damp leather. That means that you .......
a) cannot weigh down leather with metal weights, without first covering them in leather or plastic.
b) cannot have metal filings on your workbench - so be careful after you have sharpened knives where you will later work with damp leather (it takes seconds for the stains to be caused). The stains of fine metal dust will appear to look like mold spots on the leather. So also wash your hands after sharpening knives and before you work with wet veg tan leather.
c) cannot casually let metal tools lie on damp leather.
d) cannot use metal clamps to hold leather in position while drying - glue some small pieces of leather on the inside of clamps you want to use on leather.
When you spill coffee or Coke on a project, immediately take it to a basin and empty the rest of the offending liquid over the the project to cover it completely (and therefor stain it completely and seamlessly and uniformly). Then rinse it off with clean running water.
[Do not follow this tip when you spill blood on your project - 😀 ]
Always use the cut off pieces of leather to test water absorption, tooling softness, color effect of dyes and the effect of finishes you want to use. Also test out stamping patterns you might consider or any new tools you buy.
I wrote more about it here:
Remember that tools with a large footprint, will take a lot of force from your mallet to make an impression, while a small surfaced tool like a seeder, will only require a light tap.
To test the effect of a certain finish you want to put on leather, use it on a little cut off piece that was stamped/carved and dyed the same as your project and then carry it with you all the time in your pocket (with your keys). Ladies can do the same in a bag or wherever it will receive rough treatment.
This will give you a good idea of how the leather will keep, and how the finish will protect it.
I hope this helped!