This is all based on FC2-5 experiences, have not tested current rawhide.
I also don't have this in front of me so I don't quite remember
the button names.

Follow the GUI install and choosing customize partition setup,
now if you want to set up a softraid then you need to create
softraid partitions. Click the raid (or something) button and
choose the top (of the 3 possible) selection. Now anaconda has
preselected all disks available on your system, this will usually
be 2 disks in a softraid config but sometimes more.

Problem is anaconda only accepts 1 selection at a time, so I
suggest that either it does not select any or it becomes possible
to create the specified partition on several disks at once.
(If option two, the next screen could just be the FS/mountpoint
 setup screen)

A similar problem appears when you want to create the actual
raid and filesystem over your now created partitions.
You click the raid button again and select the middle option (of 3).
Anaconda has now preselected all softraid partitions and you need
to remove selections so that you only end up with the ones
appropriate for the mountpoint you now want to set up.

This all seems backward, deselecting instead of selecting.
Also it gets pretty annoying when you for example have 4-15 disks
and each has 5+ partitions. You spend an eternity just deselecting
all those disks and partitions.
(I also wouldnt mind if the partition selecting box would grow
 a little when the screen area is big enough and a lot of partitions
 are present.)