Open Minecraft, click "Options...", then "Resource Packs..." and click "Open Resource Pack Folder". An explorer/finder window should appear where resource packs are installed.

Drag and drop your downloaded resource pack/add-on file from the place you downloaded it to into the window you opened.

Don't know where you downloaded it to? It may vary from your OS and browser settings. It might be on your desktop, or in your system's default Downloads folder.

Go back to Minecraft. The pack did not appear in the list? No problem, just click "Done" and "Resource Packs..." again, the newly added pack should appear. Select it by hovering over its icon and clicking the appeared arrow pointing right. Click "Done". The game will load the resources and you're good to go.

If you also considered installing some add-ons, they usually go above the main pack. Follow the recommended selection order described on the respective resource pack page.