First, get the URL to your profile page. Go to your My Account page. Look at the existing stat button near the upper right corner. Right click on it and pick "Copy link location" or "Copy URL" or whatever your browser calls it. Paste it in below. It should look something like "http://www.geocaching.com/profile/?guid=d874252c-2b24-487e-bfed-2fb945b86f13"
Add the text you want displayed on the bottom of the button and click Submit!