Here are the examples of geojson files for Italy: https://github.com/osmlab/editor-layer-index/tree/gh-pages/sources/europe/it

Here's an example for WMS: https://github.com/osmlab/editor-layer-index/blob/gh-pages/sources/europe/it/Lombardia-Italy-CTR.geojson?short_path=feaf137

Here's the tutorial: https://github.com/osmlab/editor-layer-index/blob/gh-pages/CONTRIBUTING.md