Unpack a copy of the tarball in a private directory and make whatever changes are necessary to get the port to compile properly under the current version of FreeBSD. Keep careful track of everything you do, as you will be automating the process shortly. Everything, including the deletion, addition, or modification of files should be doable using an automated script or patch file when your port is finished.
If your port requires significant user interaction/customization to compile or install, you should take a look at one of Larry Wall's classic Configure scripts and perhaps do something similar yourself. The goal of the new ports collection is to make each port as ``plug-and-play'' as possible for the end-user while using a minimum of disk space.
Note: Unless explicitly stated, patch files, scripts, and other files you have created and contributed to the FreeBSD ports collection are assumed to be covered by the standard BSD copyright conditions.