First, copy the config plugin from this repo: https://github.com/gaishimo/eas-widget-example
You can reference my PRs there too (which, at the time of writing, aren't merged).
After adding the config plugin (see app.json) with your dev team ID, as well as a bundle ID, you can edit the widget folder to edit your code. Then npx expo run:ios (or npx expo run:android).
After npx expo run:ios, open the ios folder, and open the file that ends in .xcworkspace in XCode. Make sure you have the latest macOS and XCode versions. If you don't, everything will break.