1. Requirements documents, design document are the basic documents which you should have before starting any project. These gives you the clarity about, what the business needs to see from data.
2. While creating some dashboard on Tableau, you should be clear what is the data, from where you are bring it, joining the data, cleaning etc and then what you want to see, KPI's required, calculations.
3. When the data is loaded in Tableau and you start doing drag and drop operations, all this drag n drop is possible with the help of VizQL which translates them to trends and patterns. I would suggest you to start loading some sample data in tableau desktop and play with it.
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