Well, one important thing to keep in mind is that your stufful knows best what is and isn’t a pleasant form of affection to it. If it dislikes being touched, it may be best to begin with rewarding it or showing your affection some other way until it gets comfortable with you. Consider feeding it treats it likes or playing with it using its favorite toys or simply sitting close by it. Any of these, as distant as they seem, may be mean the world to a pokémon that shies away from physical contact.
If your stufful ever wants to be touched, let it determine when that will be. For example, if it cuddles up to you or nudges your hand, then it’s okay to touch your stufful, and even then, start slowly. Pet it, find spots it particularly likes (to have scratched or rubbed), and then move on to letting it climb onto your lap or curl up next to you. Eventually, try a hug if and only if it seems comfortable to be picked up.
I know it can be rather difficult to not be able to show the sort of affection you want, dear reader, but the last thing you’d like to do is instill resentment, fear, or other forms of negativity by forcing your pokémon to do something they aren’t comfortable doing. That can lead to mental health issues for your pokémon … and, well, physical health issues for yourself (what with the fact that we’re talking about pokémon and all). So take things slowly and let your stufful decide when hugging would be appropriate.
In other words, it’s possible to train a stufful to accept a hug, but that training actually consists of giving stufful the forms of affection it already wants until it decides it would rather have more.