There is no way to explain this in a step by step way. There are too many variables.
Biggest one is legacy BIOS/MBR or EFI install.
Roughly what you need to do is boot off FreeBSD Memstick installer or CD and create your FreeBSD partition UFS filesystem and format it.
Then extract from the memstick the files cited above. They are located on the memstick in /usr/freebsd-dist/
/usr/freebsd-dist/base.txz
/usr/freebsd-dist/kernel.txz
You need to mount your FreeBSD disk partition and extract these files onto your UFS partition.
Tar is included on the FreeBSD memstick.
Then under Linux you will need to make an entry in your grub config pointing to your UFS filesystem with FreeBSD.
Well, the fact that they are on the memstick was not obvious to me, but it makes sense to me now, thanks.
Okay, some details: I think I am using EFI as on Linux I can see that sys/boot/efi is not empty; though I am not sure still...
Now follow-up questions: I have heard that one needs to allocate a partition for FreeBSD swap, so it would be like 2 partitions at least per install, correct? I mean, can I do with having a swap in the form of a file? Or not? So the question is how many partitions should I create?
Next, do I create them while being booted off the memstick with FreeBSD image? Can I do the partitioning beforehand with parted or fdisk?
And for now the final one. It is not clear *where* to put the 2 files. I mean, I should put them to the alerady created file system that stays on one of the partitions that would belong to FreeBSD installation? Please I need more details on this one, this is also related to my first question, so maybe it is better to hear you clear that one out, and then I would ask again