Zoom in on the image to make sure that it is orientated correctly.
Find and select the image of the back of the phoneSelect the appropriate 'plane' for the image to go on (I use the 'Top' plane so the image is lying down).
Select from the top of the screen INSERT -> Canvas.
Select the rectangle and add in the dimension of 'Depth'.
Select the SOLID tab at the top, then select Extrude.
Select 'Finish Sketch' in the Sketch Palette.
You may need to zoom in or out to correctly see the drawn rectangle.
This will draw a rectangle to the size of the phone.
In the Height and Width boxes, use the user parameters 'Height' and 'Width' that you created in step 1.
Click the centre point of the screen, and move the cursor slightly to start drawing the rectangle.
Select CREATE -> Rectangle -> Center Rectangle.
Select the same plane as the image from step 2.
Create a new Sketch by selecting the 'Create Sketch' icon at the top of the screen.
Create the basic rectangle for the phone.
This will make all subsequent steps easier
Add separate entries for Height, Width and Depth of the phone.
Select the + next to 'User Parameters'.
Add some basic parameters to make the rest of the process easier.
When building a case, the first thing to complete is to model the phone itself within Fusion 360.