Regardless of your preferred approach, you always go through a series of obvious phases.
First you focus on organizing the team and work environment. You need to fill some key roles early and set up some tools.
When the team and work environment is ready it’s time to define a scope and outline a solution.
Once the scope and an outline of a solution is in place you can specify the chosen solution in more details before building.
When you know with reasonably accuracy what you’re going to build, go ahead and start.
Before finishing up, hand over the new solution to the business.