Check with your LAer about how much larger she want the back to be - usually it's 3-4" all the way around. If you are supplying your own batting, make sure it is something she accepts, or if you are buying from her inventory, decide what kind of batting you would like for this particular quilt.
I like the staystitching when the quilt has lots of seams meeting at the border. If the stay stitching keeps the quilt from laying correctly, it's easy to snip a stitch here and there. I just figure that's better than the seams splitting.
If you smoke or have pets, run your top in the dryer on an 'air' setting with a dryer sheet. If your top is well pressed before this step it will be just fine after.
Don't 'sandwich' your quilt top. That is usually done on the longarm.
Have fun and enjoy the process!!