Starts with a program submitted by the client
Then think about a general concept..."The project MUST be", "The project MUST have", "The project MUST show", "The project MUST get", "The access to the building MUST be there or here" ... Note that I'm saying MUST.. never say I WANT the project have, show, get, be... remember, it's not about you WANT, It's about the project NEEDS and that includes a construction budget submitted by the client too.
Following that, Zones.
Organize the areas, in floor plans and elevations. Every need have an space to be satisfied. And spaces can be grouped.
Then sketch a basic scheme with areas, spaces, halls. Detail what you want, think how can the project get better the environment, interiors, materials. etc...
At this point, everything is just freehanded. Not a drawn lines.
After that, Draw the floor plans and build a 3D model.
In all this process you can drive an style (Modernism, Minimalism, deconstuctivism or another you like) but keep in mind the client likes could influence in form language result of the project.
Lasting, have fun. Design could be like this, or you can just start with a plastic intention. Anyway, just find your own style and your own way. How??..... study the masters. (Meyer, Ando, Le Corbusier, Hadid, Koolhaas,etc..)