Pub 5708 is a print it out and fill in a few blocks DO IT YOURSELF guide to a WISP. There are many places that will do it for a fee. You can file without one in place, but be sure to have it on hand if an agent were to ever show up. When you got your PTIN, I believe you attested to the fact you already had one. Get you schtuff together and do something. Remember GOOGLE is your friend!