It is not really that difficult, but much can go wrong, so I suggest you back up your data ;-)
For me these steps worked quite well:
1) Install a linux distribution
2) Boot that distribution and install GRUB (will probably work with lilo too, but I always use GRUB)
3) Install all the other Linux distributions, but _never_ install a boot loader
4) Change the path in the GRUB configuration file and see if you can boot the other distributions
5) Install FreeBSD, but _do not_ install the boot loader
6)
Read how to boot FreeBSD with GRUB
7) If it works, you are done, if not go back to one ;-)
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