This Instructable uses the iPhone 4 GSM Dimensional Drawing from page 16 of the Case Design Guidelines for Apple Devices available for free at https://developer.apple.com/resources/cases/Case-Design-Guidelines.pdf
To start, open up a new file in Autodesk 123D Design. Create a new sketch by mousing over the “Sketch” menu and clicking “Rectangle.” Click the grid shown on start up to select that plane for the new sketch. To specify the first corner of the rectangle, click on the black and white circle. The dimensions for this rectangle are 58.55mm and 115.15mm.
Click anywhere to create the rectangle. It will turn yellow to show that it is a closed shape. Next, under the “Construct” menu, click “Extrude.” Type 9.34mm into the dialog box and press the “Enter” key.
In the “Modify” menu, click “Fillet.” Select the four vertical edges created by the extrusion. In the dialog box, type 8.77mm. Press the “Enter” key.
In the “Modify” menu, click “Shell.” Select the top face of the object (where the screen would be). On the far right of the dialog box, click the drop-down menu and select “Outside” instead of the default “Inside.” In the “Thickness outside” textfield, type 1.5mm for the thickness of the case. Press the “Enter” key.
You now have the basic outline of an iPhone 4 case. Next, we have to create holes for the charging port, speakers, camera, volume buttons and ringer, sleep/wake button, earphone port, and antenna.