FIRST STEP:

Head to the Splash Damage website and download Wolfenstein ET setup. Links below:

Wolfenstein ET (Windows)[www.splashdamage.com]
Wolfenstein ET (Mac OS)[www.splashdamage.com]
Wolfenstein ET (Linux)[www.splashdamage.com]

SECOND STEP:

Proceed with the installation. Installation includes ET-Minimizer(to minimize the game) and Punkbuster anti-cheat. Install game intro if you want to.

THIRD STEP:

Setup your in-game profile.
Don't forget to enable Punkbuster anticheat to be able to play on PB enabled servers(most of them are PB enabled).
You can also use different colors in your nickname. Just type ^1(1 is number of color) before any letters you want to colorize. Use this color name generator[teammuppet.eu]
If you still want to use widescreen in the vanilla ET do this:
After profile and graphics setup open the console with ~ button. Type this commands:
/r_mode -1
/r_customwidth 1920 (your custom width)
/r_customheight 1080 (your custom height)
/vid_restart
Also you might want to play with your FOV. Try different values.
/cg_fov 100
However weapons first person models will still look odd, since there is no native widescreen support in the game.
Your widescreen setup can reset if you change anything in the graphics menu. So setup everything there before widescreen commands.

FOURTH STEP:

Getting into the game. Click on the PLAY ONLINE button in the main menu.
After that you will see a server browser. You can try to find servers there.
But most of the servers are bots only. So it is kinda hard to find any populated servers with human players.
What you wanna do is head to the ET Server List[et.splatterladder.com] and find a decent non-bots servers for yourself. After that simply click the CONNECT TO IP button in the in-game browser and put your ip there and you will get into game in no time.
I highly recommend [!!!]Hirntot, 6 Map server. It features ETPro, 6 vanilla maps and stable community without any bots. IP is 193.192.59.186:27960. See you in the game!

Grab your ET Legacy Installation file from here:
ET Legacy[www.etlegacy.com]

Proceed with the installation. Install omnibot if you want to. If you don't have original ET installed, install ET assets as well.

Setup your in-game profile. As you can see there is no Punkbuster Anti-Cheat option, since PB is not compatible with ET Legacy.
ET Legacy supports different widescreen resolutions so there is no need to setup widescreen manually like in Vanilla ET. Weapons first person models will look normal, since there is native widescreen support in ET Legacy.
You can also use different colors in your nickname. Just type ^1(1 is number of color) before any letters you want to colorize. Use this color name generator[teammuppet.eu]

FOURTH STEP:

Getting into the game. Click on PLAY ONLINE button in the main menu.

Use the in-game browser to find servers. ET Legacy has in-build mod filter. Make most use of it. Most of the alive servers using ETPub, siLent, Jaymod and Noquarter mods. ET Legacy is compatible with all mods except ETPro.
Also there is bot filter in the server browser.

If you want to play with bots click on the HOST GAME button in the main menu.
This will bring you to this screen.
Setup your server as you want and then click on the START SERVER button.
In the game open the console with ~ button and type this command:
/omnibot_enable 1
Restart your server. For example change the map with referee menu. Type this command in console:
/bot addbot
This will add 1 bot on your server.
You can change difficulty level of bots with /bot difficulty command. (from 0 to 6)
Full bots command list[omni-bot.invisionzone.com]

This is all you need to know to get into Enemy Territory. I will update this guide with additional tips in the future.