You can apply for a DEA registration online at https://www.deadiversion.usdoj.gov/webforms/.  Instructions are available on the webpage. Practitioners use Form 224, the registration is good for 3 years and payment is made online with a credit card.
 
If you need assistance with the application process, call 1-800-882-9539 (8:30 am-6:00 pm EST) or email DEA.Registration.Help@usdoj.gov.