How to create a smoother edge on satin stitch

[aaahndrea]

Hi! I'm working on some designs on cotton knit tees and I'm struggling to get a clean edge on the satin stitch. I've been googling various phrases and reading blog after blog but I still can't quite grasp what's going on. Some say less density, some say more, some say more underlay, or no underlay. Oye!

It's black thread on white fabric which I know is a high contrast and probably part to blame. But basically instead of a nice smooth edge there is a jagged edge with some larger missed stitches. I'm using basic shapes like straight lines and circles so nothing wildly difficult.

Any advice is appreciated! And if you need more information I'm happy to elaborate... I just can't figure out how to attach an image.

Thank you!
Andrea