I received a divorce summons from my wife ...
In general, your wife handing you a copy of the summons and complaint would not be considered proper service of process. Did your wife hand you the summons, or was it served on you by a process server?
... I need to write my answer, per the Wisconsin Statutes Chapter 802? ... how should the letter be formatted?
In most other states, it would be per the court rules. Basically the same as Wisconsin Chapter 802. The clerk of court's site might have sample forms for that. And it's not called a "letter: it's an "answer".