If your port requires some additional patches that are available by FTP or http, set PATCHFILES to the names of the files and PATCH_SITES to the URL of the directory that contains them (the format is the same as MASTER_SITES).
If the patch is not relative to the top of the source tree (i.e., WRKSRC) because it contains some extra pathnames, set PATCH_DIST_STRIP accordingly. For instance, if all the pathnames in the patch have an extra foozolix-1.0/ in front of the filenames, then set PATCH_DIST_STRIP=-p1.
Do not worry if the patches are compressed; they will be decompressed automatically if the filenames end with .gz or .Z.
If the patch is distributed with some other files, such as documentation, in a gzip'd tarball, you cannot just use PATCHFILES. If that is the case, add the name and the location of the patch tarball to DISTFILES and MASTER_SITES. Then, use the EXTRA_PATCHES variable to point to those files and bsd.port.mk will automatically apply them for you. In particular, do not copy patch files into the PATCHDIR directory--that directory may not be writable.
Note: Note that the tarball will have been extracted alongside the regular source by then, so there is no need to explicitly extract it if it is a regular gzip'd or compress'd tarball. If you do the latter, take extra care not to overwrite something that already exists in that directory. Also, do not forget to add a command to remove the copied patch in the pre-clean target.