When you press the seam allowance to one side, there is a "low" side to the seam and a "high" side. Gently pull the seam apart and SID along the high side ON the low side right where the ridge is. When you let go of the seam, the SID will literally disappear into the seam.
When the seam allowance flips, you can gently move the seam over by a notch so the needle will land on the low side again.
THat said, I have done some work where I sewed ON THE HIGH SIDE sort of like a topstitch. It is personal preference.
When I had a little too much wine, I have meandered around the ditch (not sure they make an acronym for that - lol)