A cool, dry, dark place...
If you store or have a large # of leather/gear, I'd put a light coat of Ballistol or another well made protection/care product on it and place it in a cool, dry dark place.
You want air to circulate but for the temp/conditions not to be humid or damp.
I used to store my leather Aker shoulder rig in a old cedar lined cabinet. I'd also treat the leather 2/3 times a year to prevent mildew & rot.
As a young US Army MP in SOUTHCOM(Southern Command) I saw many soldiers, MPI/CIDC agents with leather holsers(good & bad) rot to #%*+ from a lack of proper storage & care.
Founder of Bianchi Intl and a former sworn LE officer; John Bianchi wrote a detailed guide about care/cleaning of leather products and police duty gear.