(as) easy as ABC

Extremely easy, simple, or intuitive; requiring very little skill or effort. After so many years as an accountant, doing taxes is as easy as ABC for me. Don't worry, after some lessons and a bit of practice, driving is easy as ABC! They make it look easy as ABC on TV, but it's actually very tricky to do in real life.

as easy (or simple) as ABC

extremely easy or straightforward.

From the 15th to the 17th century, a child's first spelling and reading book was commonly called an ABC , and this led to the development of its metaphorical use, ‘the basic elements or rudiments of something’.