Yeah, guess I never got around to doing that
My guide is simple. Make both disks the same partition scheme, type RAID Autodetect (except for the swap partitions) and THEN
install the system as you would normally. Slackware should recognize only one installation device: md0. You'll; have to play around with the installation location of lilo to get it to work on your setup. I believe I installed mine on the 'root partition (potentially unsafe)'.
Additional: The first time you boot it will warn you that "The RAID array md0 is not clean! Repairing" or somehting like that. You'll then notice the HD access lights going nuts for a good long time, and a process called 'raidchk' or 'repair_raid' or something of that nature taking up some CPU cycles. That's normal; it's rebuilding your RAID array, let it finish ! And don't worry, it may take a long time, but it won't take forever.
On my Athlon64 2.4 GHz with 2 GB RAM and nothing running it took something like 3 hours...