You can download the latest PlumpOS package under the terms of the GPL, without warranty of any kind, from any PlumpOS mirror. Upon downloading, unpack the archive
$ tar -xvzf plumpos-6.9-rc1.tar.gz
First let's familiarize with the directory structure here. There should be 3 directories:
rootdisk, which contains the layout of the iso to be created;
scripts, which contains small programs used to help in the creation of the iso;
final, which will contain the ISO generated by the whole process.
If you intend on using your own kernel with PlumpOS, there are several steps you must follow in order for it to boot properly. First you must make a new directory in the rootdisk/kernels/ directory that will be in the old and often blamed DoS 8.3 character format and only contain letters and numbers (and a single '.'). In that directory you should put a file named bzImage which is your kernel and a file named modules.tgz which is a gzipped tarball of your kernel's modules (with the relative path of lib/modules/ so it can be extracted from the root dir). Optionally you can also provide the System.map and config file for your kernel. When this is done you may simply run the install program and it will detect and install your kernel(s) for use with the generated ISO.
Now, say you want to include a 3rd party add-on package. This is done very simply: create a gzipped tarball containing all the files you want to include in your package (relative to "/") and put these packages into the rootdisk/packages/ directory. Then edit the file rootdisk/packages/list and add the path relative to / on the ramdisk where the package can be located (in other words for a package named "openssl.tar.gz", add the line "/cdrom/packages/openssl.tar.gz" to the file rootdisk/packages/list). Optionally you can use a line such as "cdrom:openssl.tar.gz" which will automatically search the cdrom's packages directory for the package "openssl.tar.gz". In future releases this will be useful for things like nfs and boot floppies, so for now don't worry about it ;)