I have a strange method. In general I probably have an idea what I want to keep in the tank, at least partly. Then I acquire a tank of the appropriate size. My next step is I look at a lot of tank inspirations and videos online to get an idea of what I want to do or try to do. Generally I will have one or two rough ideas in my head of what I might like. Then I start hitting up the fish stores looking for the right hardscape. What I can find will dictate what design I more or less go with. Even so, the concept evolves as I work on it.
At this point in the process much swearing and alcohol is involved. Once I am satisfied with the overall results of the substrate and hardscape I will make sure I have all the hardware (filtration, lights, heaters) and hit the fish stores or online stores for plants. As many plants as I can find and afford. So many plants. Then I'll plant my tank and fill.
This entire process can take awhile. Even though there is some frustration involved in bringing my vision to life and finding what I need to do it, the end results have indeed been well worth it.