Install a torrent client of your choice and CCG Launcher.
CCG Launcher Installer: http://ccg.launcher.eu/launcher/ccglauncher_setup.exe

Torrent the following files:
http://data.a3launcher.com/torrents/@Epoch-0304.torrent
http://data.a3launcher.com/torrents/@Epoch0310-ws.torrent
http://data.a3launcher.com/torrents/@mas18.torrent
http://ccg.launcher.eu/@AllInArmaTerrainPack.torrent

In CCG Launcher set your mod path and put the downloaded files in that folder.
In the servers list pick A3 - OP and pick whatever server you wanna play on.

pls roske l2p. arma 3 is better in every way, the physics is a bit annoying but its so that you can't make those retarded floating
fortresses. just install this shit and stop crying about how you're an arma 2 veteran and saying that it's better but for no reason.