First of all your example is not necessarily simple. It might be, but you don't know that yet.

I would draw up a list of Requirements. It would outline what is, and what is not, in scope. That takes time in itself and you should run it by the client and expand on what each point means. This will allow you to determine anything that can be removed, or needs to be added (and to just understand the project better too).

After that you can use it as the basis of an initial rough estimate.

When you have an idea of how much time it will take, add 30%.