"Stitching in the ditch" simply means that you stitch in the crease of a previous seam. The idea is that the stitch will disappear inside the seam line and will not be visible from the right side. It's most commonly used when attaching a waistband or tacking a facing piece in place.
Press the seam carefully before starting and use a walking foot if you have one.
Gently spread the fabric with your fingers or palms and go slowly.
It takes some practise but is a useful skill to have!