You actually are asking about one of my specialties.
You were on the right track. The easiest way to do this however, is to add your packages and/or remove others that you want. BE CAREFUL, however, when removing packages, as some packages are needed by others, not to mention your system needs certain ones. Then when you get done doing that, go through each tagfile, and edit it to match the contents of each directory according to what packages are in it, adding lines and/or removing lines for whatever you added and/or removed, and save it with a custom extension, as for example, 'tagfile.dot'.
Quick lesson on tagfiles:
ADD gets automatically installed
SKP (skip) gets automatically not installed
REC (recomended) and OPT (optional), when using custom tagfiles, will prompt during installation, and as I hate to sit there while it's installing packages, since I could be doing other stuff, these 2 options never get used in my tagfiles.
After all of this is said and done, build your ISO, burn it, use it, and when you're going through the install process, when you get up to the part of which kind of install, chose custom, and type in ONLY the extension (e.g. '.dot') that you used. Hit enter, and go grab a lunch, or whatever, as depending on the speed, this will take some time, especially if you only used ADD and/or SKP in your custom tagfiles, finish up your install, and you should
have a running system.