You should first "balance charge" the battery to full capacity using a suitable charger. Batteries do vary between manufacturers, and between battery types so check the manufacturer's rated voltage. Most modern chargers are smart enough to recognize the change of resistance as the battery charges to prevent over charging.
Once the battery cells maintain the same voltage, the battery is considered to be balanced and fully charged.
Now discharge the battery gently so it does not warm up significantly, until the rated cell "storage voltage" is reached. Older batteries may take several cycles to achieve a stable and even cell voltage for each cell or may not balance at all.
If an individual cell voltage/internal resistance is significantly different than the other cells, the battery can still be stored, but of course not at the manufacturer's optimum voltages. The cell to cell difference should be less than 0.1v.
You can keep the battery in a cool space at the storage voltage for more than 12 months.