Making an accessible website means more than just ticking all the boxes in the WCAG guideline. It's about making it user-friendly and accessible in practice. And confirming it through user testing.
The work on accessibility starts with the project planning - creating personas, defining user journeys and shaping the first wireframes - all of these should take accessibility into account. It continues with the design - choosing the structure, the colours, the typefaces, etc. And of course, with the implementation and testing.
Put simply, when building your website to be accessible, you just have to take a single extra parameter into account - making sure everyone can use it.