The basic steps for installing any given add-on in Kodi.
For this example a skin is being installed, the same method is used for other add-ons.
Select Install from repository -> Select the repository (if no other repositories are installed it will automatically open the Kodi Add-on repository) -> Select the add-on category -> Select the add-on -> Select Install
Step 1: Select Install from repository. If no other repositories are installed then this will open directly into the Kodi Add-on Repository. If other repositories then a different window will open allowing you the choice of which repository you want to use.
Step 2 Select the add-on category, in this case Look and feel (applies to Kodi Jarvis and up only).
Step 3: Select the add-on sub-category, in this case Skin.
Step 4: Select the add-on you want to install, in this case the skin Aeon Nox.
Step 5: A summary dialog will be brought up allowing you to see various properties of the add-on. Select Install to begin automatic downloading and installation.
Step 6: Once you select install, Kodi will begin downloading your add-on and you will see the download progress beside the add-on's name.
Step 7: Once the add-on and it's dependencies are installed you will be able to use that add-on. In the case of skin installation you'll be asked if you want to change to the installed skin immediately.
When the add-on has finished downloading Kodi will enable it and depending on the add-on ask if you want to start using it. Add-ons will then appear in their appropriate section (e.g. skins in the appearance section of settings, movie scrapers in the options of scrapers when adding a new movie source)