Regardless of your intentions, if you're registering a person's e-mail for spam, you're doing them a disservice. As e-mail usage has grown, spam has grown with it. Traditionally, people do everything they can to prevent spam from reaching their inbox. They won't be able to prevent it much longer if you sign them up for spam.
Find or collect the e-mails that you wish to sign up for spam.
Visit sites known for generating large amounts of spam. Some include AOL, MSN, Listopt, Discovery Newsletters, and Internet.com Newsletters.
Sign-up and opt-in for each of their services.
Sit back and wait. The spam will begin to rapidly flow in and continue to do so for a very long time.
While registering someones e-mail address for spam is not considered illegal, do so at your own discretion. It's not something that can just be turned off. The person will have to deal with it or be forced to get a new e-mail address. Take that into consideration before signing anyone up for spam.