First, you’ll need to get your data into a file.
Make sure your data is as complete and accurate as possible when you create the import file. Fill in any missing info, and verify that names and other information are spelled correctly.
These file formats are supported:
The maximum file size allowed for .zip files is 32 MB. For the other file formats, the maximum file size allowed is 8 MB.
If you need to import a larger amount of data, check out MSDN: Import data for additional details.
You can add multiple import files to a single .zip file, and then import the .zip file to bring in all the files at once. For example, if several salespeople enter leads from a tradeshow into different spreadsheets, you can gather them into one .zip file for import.
For best results, it’s a good idea to make sure you (or your Dynamics 365 admin, if that’s someone else) have some rules in place to reduce any duplicates when you import the data (called “duplicate detection rules”). More information: Set up duplicate detection rules to keep your data clean