Exit "SC2" completely
Download the map(s) you want to install
Make a folder on your desktop called "My Maps"
Save the .zip/.rar files to this folder.
If, when you try to download the file, it automatically "opens" it drag and drop the zipped folders into the “My Maps” folder you created on your desktop.
Extract the file - commonly known as 'unzipping'
Launch the SC2 Map editor from your start menu or Desktop shortcut
Open the map from your My Maps folder using the Map Editor
Launch the map