Easy Populate works well, but there's a trick to getting started with it. Not sure if it's mentioned in the FAQ.

Manually create a few products -- at least one for each variation you can think of. Then use EP to download the output files. Open them in Excel and study them, and use them to organize your raw data. Then try uploading a fully populated file. It's a bit terrifying the first time, but it works! ;-)

Rob