This suits me best as I see a lot of phone cases to buy that are pretty, but I like the idea of having something noone else does.
This isn't a step by step DIY but it should be pretty self explanatory - get a clear hard case to fit your phone (I got mine from Amazon), and then use a strong adhesive for plastics (mine is called Deco Princess which I got from Etsy), and get gluing with your cabochons (mine came mostly from Etsy and bits and pieces from craft stores I have picked up)! It is very easy to do - it can just take a long time! A good helper is if you have a tool like this rhinestone picker to make it a bit easier and quicker to place your cabochons.
Let you imagination run wild with whatever you want to depict with your decoden, or search places like Etsy for inspiration :)
For this case I used pearl flat back cobochons from etsy, the flower and bluebird were from Paper Valise, and the stickers are from Janetstore and feature San-X's raccoon character Kireizukin.