Design a full header, with logo, menu and banner.
1. Create a new document. I made this 700X300 pixels.
2. Use the Rectangular Marquee Tool and make a selection similar to this one, and fill it with a color.I used #003399:
3. Go to Select>>Modify>>Smooth, set the Sample Radius to 5 pixels and hit OK.Then hit Ctrl+Shift+I, then Delete, and then Ctrl+D.Your image should now look like this(notice those smooth corners):
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