Open Powerpoint, obviously :)
The first thing you need to do is change the size of the slide so that it's the size of paper so that everything prints out correctly.
Ashley Hughes at The School Supply Addict has a great tutorial about this - you don't want to squish the background!
Add some text and clip art. A lot of times I will add frames here or white boxes and layer the other stuff on top of that.
I hope that helps someone out there to create products! When I started making stuff on TpT, I never dreamed I would be successful, but I am so thankful for all the success I have had! If you have a question, shoot me an email and I will be glad to share anything I know!