After you’ve download the game, you’ll want to launch TERA_Invoker.exe from the root TERA_HG folder. It will start the launcher (none of the other binaries will launch, TERA_Invoker is the only one that works). The patch process should start automatically. Note that you may have to restart the launcher several times in order for the game to completely patch. If you get errors after verifying or trying to launch the game, first restart the launcher and make sure there aren’t more patches to download.
Here’s a translated image of the launcher (borrowed from TeraFans), to help you work your way around.
After you’re done patching, verify your files by clicking the small grey button to the bottom right of the large orange one. Once you have verified with no errors, enter your username and password and hit the large login button. Secure Login is optional, and I don’t use it, but it’s up to you.
Once you’ve logged in, hit the large orange button to start the game, that’s it!
Note that Insayne, a member of NeoGAF, has made an automatic launching program which will start the launcher, input your username and password, log you in, and then start the game (along several other things, like setting process priority for you). You can grab it off of his site. The TERA launcher does not save your username or password, and his application is nice if you don’t want to copy/paste your username every time you start the game.
Once you’ve patched your game, you’re ready to change your game settings and choose a server.