1. Consider carefully whether you do you truly love him/her. If you do, do you think they love you?
2. Talk to him/her about how feel you feel, or just show them how you feel.
3. Choose where would be a place both of you would have more comfort. It doesn't need to a romantic place; all you need to do is show them that you love them.
4. When you have thought about if he/she is the one you want to say "I love you" to, then get alone with him/her and look in him/her eyes. If you want, kiss him/her first and then just say "I love you".
Other tips:
a. Make sure you aren't feeling too anxious about saying "I love you" because you might risk to throwing up, freezing or even freaking out.
b. Make sure you look him/her in the eyes; it shows that you are in interest in him/her but it also shows that you need to tell him/her something
c. Make sure you know or think him/her feels the same about you too.
D-law