A good place to start is by reading that thread I linked to.
Specifically
Generally the more information, and the more context you can provide the better. There shouldn’t be any bugs in the game, and the game should have a copy-editor which means you don’t really need to focus on spelling mistakes either. If there’s pronoun problems, or names confused though, (like your girlfriend suddenly becoming your boyfriend) that can be a result of code so you should mention that.
You’re also looking for continuity errors, such as if you burn down someone’s house and have a huge fight with them, and in the next scene their house is still standing and they’re your best friend. Even if it seems small, mention continuity errors. (I nit-picked Zombie Exodus horribly over tiny things that weren’t all that important and I bet no one else noticed.) Also mention if you come across any sort of repetition with the same sentence being spoken twice in a row and whatnot.
Any sort of stat oddities too. If you think you should be able to do something, pick a choice and then can’t do it, mention it. (Likewise if you think you should fail given your stats and then don’t.)
Share both the positive and the negative, but don’t just say “I love this game” provide detailed feedback as to why. Maybe speak about characters you like and dislike and what it is that makes you feel that way.
Of course I’m the crazy person who just finished writing over 10,000 words of feedback on a game.