Glue the top of the box closed with a glue gun. And then take a knife & cut open, along the "dotted" lines on the front of the box. Did you know there were "dotted" lines?? When my bro & I were kids, we used to open our cereal boxes that way sometimes, slice open the bag of cereal, pour the milk right in & eat it from the box.
Fill boxes with dirt & seeds. *Dymo labels as plant markers are optional.* A Sharpie works just as swell.
I have been keeping mine on the windowsill. Make sure when you water that you have a flat dish underneath. (The water seeps out of the corners of the box.) Over time your box will warp from the waterings.
When your seedlings are big enough, transplant them to big pots! Yay for flowers! And recylcing!