Sewing "set-in" bag bottom

I am making a weekender bag that requires the bottom be set-in. It is a rectangle bottom and I just can't seem to "get it". I plan to redesign the pattern so it has an easier bottom than one set in.  Any ideas out there that will help me get nice four corners?!

Thanks,
Annette

i have made the amy butler weekend bag 5 times and the set in bottom never is perfect. It is too big and I always end up making a pleat here and there to get it to fit the bag. Either that, or sometimes I actually remember to cut it smaller.

Luckily, it's the bottom and you'll never see it again.

picture
http://www.flickr.com/photos/meringuedesigns/2211151260/in/set-72157601903227776/