Step 1: Grow a beard
You must grow a beard in order to have a beard from below. Begin the process by not shaving.
Step 2: Look up and ask your friends
Occasionally look up and ask your friends if you have a beard from below yet. Don't worry, people with beards have plenty of friends to ask.
Step 3: Take a picture
Since friends usually lie, it's best take a picture of your beard from below and send it to beardsfrombelow.org for confirmation.