The first step is to install Linux as normal. You have to leave space for the FreeBSD slice at your hard drive. You don't have to make a Linux swap partition, but if you want one, put it in the space you want to allocate for FreeBSD. That way you can delete the Linux swap partition later and use the space for FreeBSD.
When you have installed Linux you have to build a new kernel. Read The Linux Kernel HOWTO if this is new to you. You have to include both UFS filesystem support (read only) and BSD disklabel (FreeBSD partition tables) support:
UFS filesystem support (read only) (CONFIG_UFS_FS) [N/y/m/?] y BSD disklabel (FreeBSD partition tables) support (CONFIG_BSD_DISKLABEL) [N/y/?] (NEW) y
Install the new kernel and reboot. Remove any line including the word swap
from your /etc/fstab
file if you have made a Linux swap partition.
Make sure you have a working Linux boot floppy with the new kernel. Now
you are ready to install FreeBSD.