Once on the sign-up page, fill out the information as shown:
Your 'Account ID' will be auto-generated based on your business name (you are free to change it to whatever you like so long as the username hasn’t already been taken).
There will be an email sent that you can follow with documentation on how you may further set up your business account.
Note: If you do not already have a Federal Tax ID # (AKA Employer Identification Number)—please Google search “federal tax id number” and click the link that takes you to IRS.gov. From there, you will be prompted in the rest of the process.