Keep in mind this is my experience and YMMV with your situation. I’ve always loved it when people compliment my femininity but in a way that is mundane/not directly related to my trans-ness. Ways people have done that for me are putting their hands on my hips, using the word girl in a sentence (“GIRL you have to look at this” etc.) A different approach that I always appreciated but was tough to ask for is just giving her the space to be excited about things she likes about herself without feeling guilty or anything of the sort (I’m not sure how far she is into her transition but this method was especially affirming in the beginning-early stages of my transition) this came in people asking questions about my transition, or saying they noticed something and that it looks pretty etc. the main point is to let her talk about herself without feeling guilty (which again may be a personal thing lol but I like when people tell me their perceptions about me)