Methods are defined using the call:
(defmethod selector (args) code)
where selector is the name of the message, (args) is a list of the arguments of the method, and code is the Lisp code that implements the method.
A method is actually a special form of function definition. It looks almost like a function definition, except that (args) has a special form. Each argument may be either a variable name, or a list (variable class) to declare the class (type) of the variable.
An example method definition is:
(defmethod area ((self circle)) (* pi (expt (sendm self 'radius) 2)) )
This becomes:
(defun circle-area (self) (* pi (expt (sendm self 'radius) 2)) )