It's a bit more than that I'm afraid. Raid destroys the partitions and all data on the drive, then sets it up as a new volume (one the size of the sum of the component drives). You make the array by setting raid rather than ide or ahci in the bios, then hitting ctrl and i (or your boards equivalent) during post.
You then format the 300gb volume while installing windows, then copy data back onto it.
Raid is good, worth the effort. You'll need to copy your data somewhere else first sadly.
I believe it's possible to make a raid 1 array without losing the data, but I'd be reluctant to try this without a backup anyway. Hope that helps, even though it's not the answer you were after