Steam's 'phone-home' capability has been known to override only some elements of a mod, producing mixed results for no apparent reason.This 'phoning home' occurs every time the game is launched, and is independent of patches.Slightly more complicated, but lets you use the Steam Apps's online functions such as multi-player, and is only required once.

See the official Offline Mode Steam Support instructions for the type of computer running Windows OS.

Here are the very basics, but read the Steam Support instructions first.

if you have an active Wi Fi connection, your computer is always networking in the background even if the Steam App is in 'Offline Mode', so you MUST apply the hosts file fix as well): Easier to accomplish.

This prevents the Steam App from connecting to it's servers, but you won't be able to chat, play multi-player, register achievements, or update other games until you switch back to 'Online' mode.

Disabling automatic updates by itself: people have still had issues.) WARNING!

When the Steam Launcher App (the one that brings up the Steam web site, or your game library in off-line mode) is updated, it appears to reset all game specific settings to their defaults, which means it is re-enabling automatic updates.To determine where your game attempts to communicate, you want to examine your \My Games\ "Save game" synching with Cloud Storage still functions correctly, even with these addresses disabled.The XCOM-EW expansion is proof that other DNS and IP addresses can be utilized by the same or different games.For instance, EW is a DLC for EU, and as such shares files with it.Allowing one to 'phone home' but not the other is highly likely to cause conflicts with no apparent cause.Care should be taken to check the file as outlined above for changes with each patch or major release.