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 only one repository is installed, then you will automatically go into that repo) -> Select the add-on category -> Select the add-on -> Select Install
Step 1: Select Install from repository
Step 2: Select the repository you want to install from. If you haven't added any additional repos and are only using the default Kodi.tv add-on repo, then you will automatically be placed in this folder.
Step 3 Select the add-on category, in this case Look and feel (applies to Kodi Jarvis and up only).
Step 4: Select the add-on sub-category, in this case Skin.
Step 5: Select the add-on you want to install, in this case the skin Ace.
Step 6: 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 7: 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)