Mean It
First things first, and most important. Without this, these tricky words are as meaningless as the red orbs outside every Target store.
You have to mean it. You can't just say "I love you" to get in someone's pants. You can't say "I love you" just to hurt them intentionally later. And, the most tricky, you can't just say "I love you" just to gain their infatuation that you can't return.
People misuse these three words so often that they are becoming less and less valid. Trust gets lost in these trusted words everyday by lies, cheating, and leaving supposed "loved ones" in the dust.
If you feel you truly love them, and not out of jealousy, toxic attachment, or lust, then your words shall come from the heart. So stand up straight, look them in their beautiful eyes, and say the words. Go get em, Tiger.