Installing to RAIDZ1

[Baho Utot]

I want to reinstall my desktop system using a raidz-1 filesystem using 
the current installation image for usb drive.   Reading the handbook I 
find that bsdinstall will not do the installation I want.  I am using 4 
1TB drives and I want to partition to 800GB and install the raidz 
there.  Can I create a raidz storage pool manually and then use 
bsdinstall or will I have to manually install freebsd not using bsdinstall?

How do I do this?