First, this is not self contained booting. You still need the rest of the xok files accessible over NFS. However, the benefit from using this method is that you don't need an OpenBSD file system to store a copy of your xok kernel on. You can almost think of this as a network boot loader specially for xok.
You'll need a DOS floppy with nothing on it except command.com and whatever system hidden files go with it. In other words, format a floppy with DOS and run ``sys a:'' to transfer the minimum required DOS files for booting DOS.
Then place ``boot.com'' in the root directory of the floppy. This file is provided by OpenBSD for booting OpenBSD kernels from DOS. A copy has been included in the tools/floppy_boot directory of the xok distribution.
Next, once you've successfully compiled/built xok place the kernel image (sys/obj/xok) onto the floppy as well. At the moment, it's small enough to fit onto the floppy. Hopefully, it won't get too big to fit on there any time soon.
Now, if you boot with that floppy you can type ``boot xok'' at the DOS prompt and xok should load..
If you wish, you can create an autoexec.bat file on your floppy to automatically run ``boot xok'' every time. Simply create a file called autoexec.bat in the root of the floppy with ``boot xok'' as the first line.