Provost says, "I typically start by drawing a rectangle that crudely approximates the real life size of an iPhone screen, but I figured there might be a better solution that keeps the spirit of the quick and dirty UI sketch, while making sure the screen is the correct size and proportion. I decided to have a custom rubber stamp made. It works pretty well! I added little tick marks around the border of the screen so I know where the status bar, nav bar, and tab bar in both portrait and landscape should be placed."
The stamp design file can be downloaded here. It cost Provost $35 to have his stamp made at Capitol Rubber Stamp in Austin.
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