This is the way I think about seed stitch. Certain stitches “face” you and certain stitches “face away” from you. Knit and purl are basically the same stitch, except they face opposite ways. If you turn your work over, knits become purls and purls become knits.
Ribbing is nothing but columns of stitches that face toward you alternated with columns that face away from you, no matter if they were knits or purls when you made them. However, you can think of seed stitch as being less like columns and more like a checkerboard. Towards-away-towards-away-towards.
To make ribbing without a pattern, you would always make sure that the stitch you’re on is made in such a way as to “face” in the same direction as the stitch below it, to build up those columns. And the opposite is true for seed stitch – make sure you create a stitch that faces the opposite way from the stitch below it, to build up that “checkerboard.”
Seed stitch – it’s the anti-ribbing. :)