Spend some time to study the "Controls" part of the wiki:
http://wiki.esoui.com/Controls
It's not laid out in the absolute best way and is sometimes confusing, but it has (almost) all of the information you will need.
A few general tips:
In most cases you'll only ever need to call WINDOW_MANAGER:CreateControl(name, parent, type) or :CreateTopLevelWindow(name)
Created controls/windows can be accessed directly by name after they're created, regardless of their place in the hierarchy.
:SetDimensions(), :SetHidden(), :SetColor(), :SetTexture(), :SetAnchor(), :ClearAnchors(), and :SetTexture() are a few (but not all!) of the important calls for controls. (I didn't include arguments in that list in the interest of time, you can look them up on the wiki
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