It doesn't matter what you say, it matter HOW you say it. It also depends greatly on the situation. Try to gain massive inner game, then calibrate. Inner game is normally just confidence and to instinctively handle a situation.
I'd have no problem to tell a woman I like her if I feel like saying it. Even if it's the first day I meet her. Truth is that situation is really difficult, I have to admit it, but it could happen. Set boundaries for learning and break them when you are confident enough. Don't mind blowing something good, but do learn a lot when you do. Don't care about a single person too much. If you do, it should be because you are with her, not because of your need/loneliness.
First time I tell a woman I like her, is normally shortly before or after I kiss her. And I always smile. More like tongue-in-cheek smile. Even more C&F AFTER saying it.
Now, if you DO say it a bit early,
Remember how you felt when you won that basketball match when you were 16 yo? Or after your first kiss? That's true happiness. Use that emotion or anything similar to say "I like you". Better if you are laughing. Also it's really difficult that you are having THAT much fun alone, so she'll be also happy.
Remember when a person got interest in you and you felt like you would NEVER ever meet anyone like her? How she is the ONLY one who can have some interest in you? You should try not to feel like that (meet more women), and if you feel it or anything similar, that is definitely NOT the right feeling for saying that you like her. That's just you trying to rush things.
Another thing that's pretty obvious in theory but I guess hard in practice, don't give a fuck when you say it. If you say it for the RIGHT feeling (as explained before), do you REALLY think she doesn't already know? It's not SO important that you say it. Do you also know how your face changes after saying those words? If you don't have a lot of practice, it goes from happiness to deadly serious face. That's definitely not good. It says: Until now, I am "fun". From now on, I am "serious". Cheer up. She likes the fun you, she likes you, so don't act as someone you are not. Also, you DO NOT expect a direct answer (no answer assume is a good thing), so no reason to worry (;
Other thought, have a valid reason. Why do you like her? No, don't say because she's damn hot. If you happen to really say it too early, your brain has to find really fast a valid reason and join everything into a sentence. Even more if you don't know her feelings for sure. Not the best, but will save your ass a couple of times. "I like you [dead silence]" is much worse than "I like you, you remind me to the brat little sister I never had

".
EDIT: regarding what other said. Realize how they assume that, after you say "I like you", you just become passive while waiting that she likes you. That's one way, the way of saying it and being serious afterwards. The way of saying it and giving her full power afterwards. If you say it and don't care or even make it fun, you retain the power. If you say and she's not sure how much you mean it or even if you DO mean it, while she probably feels a bit of it, you retain the power or could even get some more. First time(s) say it as a compliment, not as a marrying proposition lol. It's your waiting/expecting face what gives her power, not the sentence itself. THOUGH, this is 100% accurate IMHO "when she asks you if you like her, you just respond 'You're pretty cool.' ". That's almost always a proper shit test.