Care: Togo, Mill and Nappa one by one
Three finishes, three needs. The most common mistake is the same for all three.

Konstantin Evdokimov / Unsplash
First the mistake almost everyone makes: too much care, too early. Vegetable-tanned leather leaves the tannery with enough oil in it. Saturating it with balm on day one prevents the very patina you wanted. Only condition it when the leather looks dull and feels dry. Under normal use that is once or twice a year.
Togo
The lifted grain catches dust. Dry brushing with a soft brush is usually enough. Condition sparingly and work the cloth into the grain rather than across it, or the oil settles in the valleys and dries pale there.
Mill
The structure is deeper, so the same rule applies more strongly. With Mill it is worth opening the folds before you condition. Oil left sitting inside a closed fold becomes a dark line the day that fold opens.
Nappa
The most sensitive of the three, and the only one where you should test first. Nappa has no coating: any product sinks straight in and stays. Try a hidden spot and wait a day before treating the whole surface.
Water
For all three: getting wet is not an emergency. No hairdryer, no radiator, no sun. Let it dry at room temperature, shaped the way you want it. The dark patch goes away. A leather dried too fast turns hard, and that does not go away.