The stitches should butt up against one another. This frequently means going no further than one thread of the ground. The outer edges should form a smooth line. This is a tricky stitch to master and there are many methods to do it. Sometimes you will see directions for making a running stitch outline, but this can actually cause more problems - particularly for a beginning stitcher.
Satin stitch takes practise.
In time you will master it. First, work a row of stem stitch. Whip over the stitches without picking up the ground fabric. In very fine embroidery, use more than one whip stitch for each outline stitch to produce a cord-like effect. For better illustration, the example shows a darker thread for the stem stitch, and a lighter thread for the whipping.