The quilt top has 12" squares with alternating 4 x4 12" squares. There is sashing with cornerstones surround the blocks.

I want to do some stitch in the ditch. Do I use a walking foot or a stitch in the ditch foot? Do I stitch top to bottom and side to side in one direction or do I alternate?

I prefer the walking foot. I stitch my sashing in the same direction top to bottom & same for the width.

a walking foot and a stitch in the ditch foot are the same thing-
a darning foot/free motion foot is the other quilting foot-
use the walking (sid) foot for straight lines-
use a darning(freemotion) foot for curves- free motion quilting-
normally quilting is started in the center- work out toward a side- out toward the other side- out toward the top- out toward the bottom- evenly moving out to keep everything even and wrinkle free.