UI's you have to love them, but how many people know how to make a good (not messy UI) that works? Not that many..
Even though it is a custom UI that you made it still helps to have a clean UI. By clean I mean understandable by other players other than you.
To make a Custom Clean UI you well need:
Movable Action Bar Addon (Bartenders recommended, or Bangos)
Movable Player Frames (Xpearl Recommended)
Cycircled is a PLUS
-Any other addons that you find might help you, just browse Curse.com.
Making your Custom UI:
Now it is time to make the UI, I recommend messing arround with Xpearl and Bartenders to understand them (They have really easy options that a chimp can understand).
Ounce you get the hang of the controls simply make it. It should be equal on each side if you want it to look clean (Ex if you put a bar on the left side of the screen make a bar on the right side).
Customizing UI Depending on Playing Style:
Well many classes have spells that can save you butt at the last second, like Fire ward/Frost wards for mages, Feign death for hunters etc. Make these really easy to hit.
If you are a clicker make the main spells really big, this well help you allot. If you are a keybinder make all the bars really small that are key bound and make the buttons you click really big.
If you are a Rogue for example make your opponets frame big, if you are a healer make your player frame big and your targets frame smaller.
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I have made alot of custom UI's I think they are really fun to mess with. My first one was a mess, the best tip I think is to make everything semitricle (even).
Also I am now using Spartan's UI, I really recommended it. I got sick of re updating my UI's =[, but they are fun to make :P
Spartanui.com