This tutorial describes in detail the process of designing and 3D printing and iPhone case with a GoPro mount. I have attached a video link for an more in depth look of all the steps.
Step 1: Creating the Phone Case Body
To start off, I created two intersecting sketches as shown in the first picture. One lies on the front plane and the other on the top plane. The first sketch, shown in picture 2, is drawn on the front facing plane, and is the outline of the iPhone. The second sketch, shown in picture 3, lies on the top plane, and this is the outline of the phone case. NOTE: It is important to make sure these two sketches are touching each other.
Create a swept boss/base using the two sketches created.
Step 2: Creating the GoPro Mount
To create the GoPro mounting prongs, create a new sketch on the right plane (it should be centered horizontally to the phone). Sketch out the shapes shown in picture two with the same dimensions. Proceed to do an Extruded Boss/Base midplane with the thickness shown in picture three.
Start a new sketch on the Front Plane and dimension the shapes as shown in picture three. Do a mid plane Extruded Cut through the entire solid. This should create two cutouts, from front to back, as shown in picture one.
In order to hold the hex nut in place, make an Extruded Boss/Base on the right side of the extrusion just made. Reference picture four when drawing the hexagon and the circle.
Step 3: Volume and Lock Button Cutous
Create the cutouts for the volume and lock buttons, by creating a sketch on the left end and right end, respectively. For volume cutout dimensions, reference picture one. For power button cutout dimensions, reference picture two.
Step 4: Headphone Jack, Power Port, and Speaker Cutouts
Pictures one, two, and three show the dimensions for the headphone jack, power connector, and speaker cutout respectively. The three cutouts are made by making a sketch on the bottom plane and doing an extruded cut long enough to cut the bottom wall.
Step 5: Exporting
In order to 3D print the phone case, it needs to be saved as an STL file. It can be done by going to File>Save As and slecting .stl and the output format.