Not sure how to answer that question, but I will tell you what I have done.

You start with what you know and keep adding detail in your design untill its done. Think as much of it through as you can adding details like steering box, gas tank, shocks, everything. Think and imagine as much as you can. Sketch some stuff. Measure some stuff. Prop some stuff up and stand back and look at it.
Eventually it will become reasonably clear what you have to do and what the frame is going to look like. There are thousands of possibilities. Whatever you want (within certain limits) is possible.

I like to think in terms of closing in on it. Lots of backing up along the way. I will think its going to be like this, then my buddy comes in and asks me where I plan on putting the gas tank. Opps, forgot about that. Back to the drawing board.

I am in the middle of building my first car. I have built a lot of other stuff and even some vehicles but this is my first ground up chassis. I am having a blast.
There is lots of great information here, learn to use the search function, its like being at the library of car builders. I would not be anywhere near as comfortable as I am without all these guys help.

Good luck. Post some pictures.