Disabling firewall and installing some VPN - totally safe :D
Anyway, just install the multiplayer client and run the game using the multiplayer shortcut created on your desktop. That's it. Oh and this is Crysis multiplayer, not Crysis Wars multiplayer. For Crysis Wars check
https://crysiswarsmp.com/ or
https://cryserv.de/
About CW maps in C1, it depends. Some of them work, some of them don't. Many of them require additional manual edits. Some of them have been ported to C1 already.