The key to a decent SID is:
1. a well-pressed back (hopefully with the seam on the same side.
2. a nice taut sandwich so there isn't much puckering.
3. starting in the center row and working out from there.
4. sewing on the low side of the seam and pulling the seam line apart slightly so you can get a good sense of where to sew.
5. going slow and steady.
6. when the seam changes direction, simply let the needle move over (it's only or or 2 stitchs) to the low side of the seam and continuing on.
7. using a walking foot helps but isn't necessary when the other steps are followed.