matt2223,
have a look in the Book Amendments & Extra Guides section (I presume you havn't read it):
http://www.haynes.co.uk/forums/showthread.php?t=2430
Print off the document and take in to the workshop. It's what I did.
Following the guides, my first attempt was reasonable but not accurate enough, (I cut the vee for the bend a bit too big and after bending it ended up with too big a gap to weld), so made another. Talon's tip is sound and it's what I did.
It's tricky, take your time, think thrice, measure twice and cut once, as the saying goes.
You certainly aren't the first and definitely won't be the last to have a bit of difficulty. It's probably the trickiest bit of the chassis for those cutting their own metal.
Oh, and no matter how accurate you are, the suspension braket that fits to it will need a little packing, see:
http://www.haynes.co.uk/forums/showthread.php?t=6709.
Good building,