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The first step isn't design or layout, it's interaction. You should do many interaction-only prototypes with NO design and NO layout first. When you understand the core interaction flows, some good candidate layouts will be self evident, and you can iterate on the design.The most valuable thing your early prototypes can tell you is which features you can get away with leaving out for V2. You probably only need three features that can be built with 100 lines of code. The problem it's it's extremely difficult to understand what those 100 lines are. Most companies spend millions before figuring it out, but you don't need more than pencil+paper to do it.
You really don't need CSS until after your first sale.
For me Pidoco is working best for the wireframing and low fidelity prototyping stage.
Find some drawing program that is free and within their hardware constraints.
At the mock up stage, I think the most important piece is hammering out effective communication protocols. Regardless of what tool you use, this will be a process. It sounds like you may be in different countries. If so, you may find that bridging the culture gap is harder than bridging the tech gap.
It allows you to rapidly wireframe UI / UX, and then prototype interaction flows by linking views together.
There's also PoP, which is an app for iOS, which allows you to do the same thing with photos you've taken on your phone.
Most people already know how to use them and you can create pretty realistic designs using them!