How do you tell your best friend's cousin that you like them?

It depends. Do you REALLY love her? I f not than don't because she can feel the same way the if you two break up that would be one awkward family reunion. But if you really do love her and want to spend the rest of your life with her then this is what you should do:

1.) Find out how she feels about you.

2.) Find a time and place to tell her.

3.) Stare her straight in the eyes.

4.) Just lay it out in the open.

5.) Await her reaction.

But be careful if your family finds out they might get a little worried. Just remember this- you will have to tell her some time or it will eat at you forever, over 20% of people are married to their first cousin, Einstein married his first cousin, having a child with them only increases the chance of mental/physical problems by about 2-3%, their is nothing biblically wrong with it (The Bible says not to marry practically every member of your family but not your 1st cousin), people WILL ridicule you but just ignore them, get her to hear you out even of she wants to run and scream, you have to tell her face to face, some states don't allow first cousins to get married, and I'm going through the exact same thing but my cousin lives in a different state so i can't tell them right now. I wish you all the luck in the world dude.