Step 1: research on the ISS focused on biosphere components and food production.
Those aren't baby steps - your step 1 is no lower than about step 5 in any rational plan. We don't even know how to build a biosphere _on the ground_. Baby steps start with the basics, not three quarters of the way up the curve in the most expensive place to perform research.
At the same time, work on high efficiency, low reaction mass propulsion systems.
We already have those. The problem is, they absolutely suck because high efficiency and low reaction mass means absurdly low thrust. (F=MA after all.) Absent new physics, that's not going to change and such drives are going to be useless for manned expansion.