Telling your best friend you love them can be a little tricky
For starters, it is very important to know whether you are truly in love with your friend or is it just attraction due to the amount of time spent together. If your feelings are clear and you are indeed in love, think of a good plan about how to tell them. Pick an appropriate time and place to do so. Do not burst out emotionally when you are out with him/her, because he/she is going to be taken aback. Do it in the comfort of your home, rather than in a public place. Be clear and honest about your feelings. Consider the fact that your friend might want time and wouldn't want to be close to you for a while after the revelation. Give your friend the chance to make a decision and respect the decision, whatever it is.
Alternatively, you may want to spend more time developing your relationship if the feelings are shared but would be content to stay friends as is if not. Once you are sure, leaving a few hints here and there would certainly help. Going for romantic movies and dinners, buying gifts, hugging good-byes; these are some signs that you can work on. But remember, take it slowly.