The famous smooth edges technique Tutorial
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Intro:
This adobe photoshop tutorial will show you on how to make really smooth edges for future use in all your photoshop designs!.
Step 1:
Make a shape you like, I used the polygonal lasso tool.
Step 2:
Ctrl+Click the layer you want to smooth, this makes a selection around the shape.
Then Hit this icon:

What we just did is we made a selection, after that we make the layermask, the selection is needed because without it we just have a layermask that has no black and white, just white. White is visible and black is not, so we won't accomplish the effect.
Step 3:
Alt+Click the layermask! This makes the layermask visible instead of the original image.
Blur the shape with Filter > Blur > Gaussian blur, and with a radius of 4px.
Step 4:
Press ctrl+L to popup the levels menu, and add these settings, if you have another shape just play around and if you don't just play around too just to see how it works..
Input levels: 110 - 1.00 - 151
And that's it! I can garantee you if you ever need round edges this is the technique you HAVE to use!