Typical charger will boot up in "charge" mode for Lipo. Use the right arrow ">" key until you see "storage" and then hit start.

...Be sure balance plug is attached...

it will flash on mah, most will have a very low value there, use the right arrow key again to push that number, likely it only goes to 2 , in 1/10 increments. Go all the way up and hit start again, this time it will be flashing on the number of cells, arrow left or right until you have the number you need based on the battery you want to storage charge.

Most chargers have a safety timer so that after 120 minutes they just stop. If the battery is a large capacity one and you can only pull 2 amps out with your charger in storage mode you may have run it more than one time.