I need to shrink my lacoste shirt?

I ordered a lacoste shirt online, but it is too bug they will not let me send it back, so I need to shrink it. I usually wear xs or s at Hollister and American eagle, and this lacoste shirt is s (4). It is a regular shirt with the crocodile, and no graphics. It is 100% cotton, and it says wash in warm water do not tumble dry and iron medium heat. How do I shirnk it? Thanks!! The person that gives the best answer that actually works gets 10 points :D