There is a Windows and a Mac build for Blender 4.1/4.2 Linux has not been tested and should be treated as non-compatible

Just like any other addon. Drag in the .zip into your .blend and hit install - it is normal for this to take about a minute.

Under the hood this uses the imaginairy-normal-map ai implementation. This can take some time depending on your specs. Obviously larger images take longer than smaller ones (very large images are resized to not destroy your graphics card). The first generation will take the longest.

Think of size being quadratically related to speed. So doubling the x-resolution will quadruple the processing time. Oftentimes it's fine to use a smaller image anyways for just general surface normal directions.