Define parameters: define parameters in the function, and some parameters can set default values. For example, the chamfer command may require two parameters: the object to chamfer and the chamfer radius. You can set the radius parameter as optional, and the default value is 1 unit.

Handle parameters: inside the function, use the (setq variable value) statement to handle the passed-in parameters. For optional parameters, you can use a conditional statement (such as if) to check whether the parameter value is provided. If not provided, the default value is used.

Call function: Now, you can execute the command by calling the function. If no parameter is provided, the function will perform the operation with the default value. If you need to modify parameters, you can provide new parameter values when calling the function.

Through the above steps, the CADLISP command with default parameters can be realized, and the parameters can be modified as needed, similar to the chamfering command that comes with CAD. Please note that the implementation may be different due to different CAD software versions and related LISP environment.