@azzoor has this great video with some advice on how to set up your device for testing accessibility and a ton of tips will get you testing effectively in no time.
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Manual testing is crucial. And therefore, reducing friction to let you start your testing process can be a huge help. Selecting some accessibility shortcuts will do that, putting most of iOS' accessibility features at a triple-click of a button.
If, for some reason, you are creating a button from scratch, instead of relying on UIButton (perhaps you are adding a fancy micro interaction animation?), take into account that you’ll need to configure the button accessibility trait.

Sometimes your UI will just not scale for large text sizes. Simple changes, for large sizes, like disposing elements vertically instead of horizontally, reducing the number of columns, and allowing more lines of text, can do the trick most times.
Content © Daniel Devesa Derksen-Staats — Accessibility up to 11!