Use an infused pouch (eg. Spirit Wolf pouch), right click the item on inventory, then press "summon". You can only summon familiars and pets at your Summoning level.
Requirements: Completion of Wolf Whistle
EDIT : For followers that last until you shoo them, (e.g dog* or cat,) you right-click on them in your inventory and click either "drop" or "release". (depends on the pet) Remeber to feed them!
(*Dogs also require completion of Wolf Whistle)