How to paint baseboard without painting the base shoe?
It would be quick and easy to mask the shoe. Slide a roll of high-quality painter's tape along the baseboard, tight to it, while pressing the dispensed tape against the shoe. Run a finger wrapped in a thin towel along to press it tight.

Pull the tape shortly after painting and scrape off any bleed with a soft (plastic or wood) tool while the paint is still soft. (Pulling tape quickly also results in softer, less-conspicuous paint edges, as the edge can contract minutely as it dries.)