Preparation has been superseded by Pipeline Builder and is therefore no longer the recommended approach for cleaning and preparing data. Pipeline Builder makes it easy to clean and prepare your data for pipelines, while also offering Marketplace support.
This page will help introduce you to the Preparation interface for cleaning and preparing datasets.
Choose Clean in Preparation from the Actions menu on any dataset to create a new preparation for that dataset.
Click the Select a dataset... button, and select the dataset to clean/prepare.
Click the Save as dataset button in the header bar.
Select where you want the dataset to be saved, then click Save. The dataset will begin building, and you will be notified when it is ready.