Pick whichever matches your screen resolution - if you're unsure, try them and see which looks right. To install: From the Game Menu, hit Edit Mode. Click the Layout dropdown, click Import and paste one of the below strings.
The sanest thing to do is to recommend Luxthos' - no matter your spec, you can use them, because he makes them for everyone. Just be wary they may not be 100% perfect because of the same-design-for-everyone approach. Try out other ones if you want!
Type /details options in game, then click Profiles on the left, click Import Profile and paste the above string! If you want all your characters to use it, give it the name "Default".
Install the above addon, then type /details options in game. Click Skins on the left, then under Skin Selection, pick "The War Within" from the dropdown.
Type /plater in game, then go to Profiles. Click Import Profile and paste the above string. Enter a name for the profile at the bottom - if you want all your characters to use it, name it Default and override. It may warn you there's no Wago information - that's okay, just go ahead.
By default, DBM makes a lot of sounds and flashes your screen. Helpful, but I prefer playing without those. If you want to try it, import this DBM profile. It disables almost everything except the timer bars.
This is all you'll get. This, and chat frame messages.