Here is a video that will explain how to start your design on Sketchup. Research phone cases that exist today, some are better than others to protect your phone. In order to build a case that will fit perfectly on your phone, here are the precise dimensions for the iPhone X. Make sure you follow this dimension when designing on Sketchup.
Dimensions for the iPhone X:
height:143.6mm
width:70.9mm
depth:7.7mm
In order to help you with your research, here is some company that makes phone cases; Otterbox, Spigen and Mouse.