READ THIS BEFORE DOWNLOADING
Installation: Once a pack has been downloaded, extract the .rar into your osu! Songs directory. All songs are still .zip'd and/or .osz'd inside the pack, so osu! will need to extract the beatmaps itself the next time you go into Play mode. Do NOT extract the zip's/osz's yourself, or the beatmaps will display incorrectly in osu and will not function properly.
Also note that it is highly recommended to download the packs from latest to earliest, since the oldest maps are of much lower quality than most recent maps.