Smoothing Issue

Xets92

Hi, im having trouble with small smoothing issues at the edge of a simple model im working on. I have watched a few videos but im still struggling to understand what causes this. I'm trying to get a smooth look like on the longest sides of the model, with no hard edges.

Any help would be much appreciated, thanks

Screenshot is attached.

SnowInChina

make control edge to the inside with even thickness (quads only)