For Datawrapper, Lisa Charlotte Rost outlines the steps to prepare and clean your data in Excel or Google Spreadsheets. From the beginning:
When you download an Excel file, it often has multiple sheets. Our data set has three of them, as seen on the bottom: “Data”, “Metadata – Countries” and “Metadata – Indicators”. Look through all of your sheets and make sure you understand what you’re seeing there. Do the headers, file name and/or data itself indicates that you downloaded the right file? Are there footnotes? What do they tell you? Maybe that you’re dealing with lots of estimates? (Does that maybe mean that you need to look for other data?) If you don’t find notes in the data, make sure you look for them on the website of your source.
The guide is in the context of prepping your data to load into the Datawrapper tool, but the advice easily applies more generally.