Well, everything is explained in tooltips also theres Tutorial button if you need.
It goes something like this
Define base item - skeleton/template. This item can have base values like 10 damage etc. Also it defines certain aspects like button or class.
Optionally at this stage you should define any attribute behaviors you want to use.
Define base growth if you need. This will cause item to 'grow' in stats with each level.
Define list of Prefixes and Suffixes. Those are created for each possible combination of item unless they are filtered for defined conditions.
Define list of Tiers. Those are multiplers for item's stats. For example 'Weak' item can have 0.7 multiper for stats were 'Epic' 2.0 etc. In addition those define item color name and global actor per tier.
Configure how you want all stats to be named, how to display thier values in tooltips etc.
Now you can either copy paste XML Data (right click to avoid paste limit) or use Inject to save directly to your map.