Where is an adverb put with these expressions?

Bonjour Antoinette !

This is a trickier question than it looks!
It depends on whether you're describing the whole action of hurting, or if you're focusing on the level of hurt you're inflicting.
Indeed, remember that the expression in French is literally "to do harm/hurt to someone".

1- If it's the whole action (often, never...), then the rule is as usual with adverbs:
- in simple tenses, it comes after the verb =
"Tu me fais *souvent* mal. / Tu fais *souvent* du mal à ta sœur."
- in compound tenses, between the auxiliary and the past participle:
"Tu m'as *souvent* fait mal. / Tu as *souvent* fait du mal à ta sœur."

2- if you're focusing on the level of hurt you're inflicting, therefore qualifying "mal" =
- With "faire mal", the adverb will come before "mal":
"Tu m'as fait *très/extrêmement* mal." (You hurt me very much / badly.)
"Il me fait *vraiment* mal." (He really hurt me.)

- With "faire du mal", because here "mal" is a noun, you will use "beaucoup de" (a lot of "hurt"), etc:
"Il me fait *beaucoup de* mal." (He hurts me a lot.)
"Tu lui as fait *moins de* mal que lui." (You hurt her less than him.)

I hope that's helpful!
À bientôt !