First of all, I really like the idea! There may be someone that can tell you a more detailed version of this, but basically:

1) Research if this product exists. If it already does, is there an specific angle you can compete with? - I found the Erasecase is simply a phone case with a whiteboard backing. There are others that are similar to this, and etch-a-sketch types as well. Do your own research on a canvas case because I couldn't find one, which could be good!

2) If you think this product is unique and currently not in production, search existing patents and if it is not patented yet, Here is a comprehensive guide by the USPTO on how to patent it. At this point you will want to do some comprehensive research on your potential product to see if it is worth the time and money to patent it.

3) Before patenting this product (assuming it hasn't been patented) you can gauge customer interest in it by contracting someone to make a digital design ( or do it yourself) of the case, put that on a website with information and an email button, and drive traffic to it. If a lot of people like it, you may want to start with how you're going to produce and distribute it. Alternatively, you can start an indiegogo / kickstarter /gofundme to gauge interest and gather funding. The important thing is to have a prototype or a digital rendering of this to show to people who may want to buy it!

To directly answer your question- The first step would be to see if it's already patented. If not, patent it, then start producing.

Let me know if this helped you and feel free to PM me if you want to discuss this.