I'd like to remember that Acting Lessons is a Visual Novel, and it's heavily story-focused. I don't want to spoiler too many details even if I am submitting a guide. So, I will limit myself to tell only what you have to do, without telling important choices you have to do during the story.
To unlock this achievement, it's necessary to focus on Melissa since the start (you first meet her with Megan, in a cafè). Whenever you spend a moment alone with her, or converse with her, be gentle so that she understands that we really care about her; although at first she will always seem to have some problem with the way we do things (she says she feels uncomfortable about a romantic relationship, and that she just wants to have sex for fun). To have a better chance of ending the game with her by our side, we must avoid falling in love or at least remain just friends with Megan and Rena.
A tip: before any choice (minor or important), always save, so you can reload the game in case you think you've done a wrong choice.
I'd appreciate it a lot if you can put a thumbs up to this guide, if it helped you! And I'm open to write any improvements to this guide, if you think I missed something! :)