Go onto the Apple website and find the technical drawing to which model you want to design your iPhone case forhttps://developer.apple.com/resources/cases/
click the top .pdf and scroll until you find the right one.
After I got all of the measurements from the drawing I simply went into illustrator and went to the rounded rectangle tool and clicked on the screen to input the measurements. I believe for the 4S I used 9mm rounded corners.
After I had the initial shape I located the buttons around the side of my phone and decided how big of a gap I wanted to leave on the side for them. All I did to do this was make a rectangle, input the dimensions like the rounded rectangle, laid it over the initial rounded rectangle vector in the appropriate place and went to the pathfinder menu > unite
I then made the shape for the camera, and 2 more vectors scaled very slightly than the original rounded rectangle to create toolpaths, and to cut out the case.