Shouldn't have to do anything other than use the correct filename extension (and write WML files, for course). Apache on my Mandrake box came preconfigured with these mime types:
application/vnd.wap.wmlc wmlc
application/vnd.wap.wmlscriptc wmlsc
text/vnd.wap.wmlscript wmls
text/vnd.wap.wml wml
If yours doesn't have this info, find the mime.types file that Apache uses and edit it.
Of course, XHTML-MP is the new WML. :-)