The methods of serving a notice are contained in WI Stats. §704.21.

The hardest to challenge, and therefore often preferred method, is certified mail. Remember that you must add two additional days when you use certified mail

Other methods:

• Personal service. Giving the notice by hand to the tenant or a member of their family over 14 years old in person. This sometimes results in an opportunity to come to an agreement, but it is difficult to prove that the notice was served. Owners have been harmed when serving notices
• Substitute service. Handing the notice to a competent person who is in apparent control of the premises. You must mail a copy regular mail.
• Posting a copy in a conspicuous place and mailing a copy. This method may only be used after reasonable diligence getting personal or substitute service. Milwaukee County interprets this as a minimum of three attempts over two days.

To learn more we highly recommend looking at the Apartment Association's Landlord Tenant Law Boot Camp