I think you meant freezing for bait, well you can freeze it whole or fillet it and save space in freezer.
firstly, make sure its clean and dry, wrapped seperate in cling film ,makes easier seperate when you want fillet or two when going fishing.
most of all freeze soon as possible after caught take a cool to keep it fresh
as when to use as bait , any time you want as it is another option to use by its self or use as a cocktail with other baits