Introduction to the architect's toolbox
Architects in the physical world don't just have an idea, grab a hammer, and immediately start building. This is for good reason. To do so would fail to account for a large number of valuable intermediate steps that are necessary to ensure the validity of the final product. Missing, eliding, or otherwise deprioritizing these steps violates the generally accepted practices of the profession and would almost certainly result in an edifice that is suboptimal, unsafe, or unfit for purpose.
Security architecture is no different in this respect. There are a number of key steps involved in the development of a security architecture. Each of these steps can be thought of as a specialized tool to validate the end-state design in a very particular and discrete way. Obviously, not every tool will be applicable to every situation. However, building an understanding of what tools there are means that you can select the right one for your purposes...