Things you'll need:
The bed (Obviously)
Wood saw - just a bog standard one will do
Bit of sandpaper
Optional: If you want to make life easier for yourself, then I'd recommend a Kreg Pocket Jig. The starter kit can be found on Amazon or Ebay, comes with some decent instructions and a selection of screws (though you don't really need the special fittings). More on this later.
Step 1: Bed Ends as Sides
First step is to decide how wide you want it, then measure that back from the edge of the side pieces. Don't forget to also allow for the width of the rear posts so you get it right. I forgot on my first measurements and I'd have ended up with a thing that wouldn't fit through the greenhouse door. And don't forget to use the angle on your saw to make sure its straight.
Make sure you've got the size you want and you're good to start fixing it.