Compliment her on everything every day. Tell her her hair looks great. Her dress is pretty. She has a good sense of humor. Be nice to her. Every next day she would feel seen and appreciated. Kindness is great. Don't criticize her. If you do not like something, stay quiet. If you open you mouth, say only good things.
until your compliments become redundant,
until she thanks you before you even compliment her,
until she does not notice that you even complimented her,
until she starts to ignore you because your compliments are meaningless.
A compliment has any meaning only so long you don't compliment or disparage people, too. To encourage and to discourage go together, just like high and low, full and empty, inside and outside. Without depreciating, you are fundamentally incapable of appreciating.
In fact, you no longer even know the difference between appreciating and depreciating because appreciating is all you have ever done.
Before you know, your compliments have become 8th prize trophies. You have used them rather abused them so lightly and abundantly that you have actually become insulting. She can come out looking like a wreck, feel horrible about it, and yet get complimented.
Now, no girl can even feel good about her because all people have ever done is thrown compliments at her. No matter how genuinely you tell her how good she looks, she thinks that you are just following common courtesy. This is called a toxic culture.
What then should we do? Tell the truth. When her hair does not look good, say that. When her shoes don't go with her dress, say that. When her hair does look good, say that. She can now feel genuinely appreciated because you did not say it the last time. Now, it becomes genuine because your compliment was used in relation to its contrary which gives it its essence. You can't have one without the other, just like you can't have full without empty. Only when truth and its free expression (honesty) are the norms can your compliments ever mean anything.
When everyone is nice and kind, the entire world can be made insecure.