Installation:

1. Install downloaded addon .zip file like any other addon for Blender and enable it
2. In addon's settings, choose initial resolution or leave it as 1K 
3. In 3D View open right panel with shortcut 'N' and find HM tab, if you can's see an addon first select any geometry in your scene
4. Click on Sync Library button and wait around 2-3 minutes. Over 130 hdris will be downloaded in the background and library assets will be created
5. Open Asset Library and find library named: hdrmaps
6. Drag and drop on the 3D Viewport any hdri asset
7. You can tune any hdri asset with HMTools: add blur, change exposure, contrast, rotate hdri or even add flatten ground option.
8. HMTools Tweak panel also works with any hdri added to World, not only with HDRMAPS world asset
9. You can save all hdri tweaks and change other resolution to asset file.
10. After you click Save HDRI Asset, any missing hdri file will be downloaded from API, in just few seconds, and environment asset will be recreated with new settings and hdri resolution

Licensing:

Important! Although an add-on is released on GPL terms, please keep in mind that all free hdris that you can download with it are licensed as Royalty Free