Here is a link to a template file to create a draft class. The file is called PBdraft.db and can be opened with a database editing program such as SQLite Studio.
An important thing to know is you're going to need to keep a copy of any draft classes you create on your computer somewhere - the game will only recognize the file to import if it is called PBdraft.rkf so you're going to want to create a file and save it as say 2003.db, 2017.db etc... and then when you want to use one copy and paste it and then change the name to PBdraft.rkf
The PBdraft.rkf file must be placed in your league's save game folder. The path you will want to look for in the directory where the game is installed will be the app/disk/saves/yourleaguename folder. Place the PBdraft.rkf file in that folder. Then proceed to the popup where you will choose the FILE IMPORT option.
It's always a good idea to save the game and keep a back up your files before importing the rookies the first time you to it just to make sure nothing goes wrong.