I got ferret malt paste (can get it pets at home or amazon) . Put a bit on my finger and they lick it off . When they have a nibble on the finger I push finger in their mouth . Did that every day regular and in the end they realised biting the finger wasn't fun. Also i picked them up loads. Even though it seemed over the top , if I was with them a hour I'd pick them up and put them down every few minutes. Just trying to get them used to being picked up.