After the bunny is tamed (via inserting carrot in Nishy's lower entrance then luring the bunny to the green taming and letting the bunny lick the bottom part to get the carrot ), you can start a HAct by crouching (hold C) and interact (Left Mouse Button) with the fapet/bunny, you must be near her and facing her.
To make her follow you : after starting an HAct you must hold the 0 key (on english keyboard it's above letters O and P), a white bar in bottom left corner will appear and will start filling up. Keep holding until it's filled, then the HAct will be canceled and the fapet will follow you around.