how can i get these smooth edges?

[RetroDesire]

in the picture how can get my edges similar to the ones like this?

[Lisbon]

"how can i get these smooth edges?"

By blurring them.
Here is one way.

Start by loading the shape/layer as a selection (Ctrl + Mouse click on the layers thumbnail)
Convert the edge selection into an outline (Select > Outline / Radius=2px)
Finally, add a live Gaussian blur (Radius=1px)

Note that I have reduced the opacity of the blur. This helped the shape blend in better instead of just looking blurry around the edges.

The most important thing is to get an edge selection. Affinity Photo v2 allows band pass masks. I've never actually used it, but I think it might work.