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.
Step 2: Fixing up joints
Whichever way, fix the side pieces to the back and you've got three sides done already
Step 3: Add front bar
Step 4: Adding the work platform
Step 5: Adding a shelf
Final and optional step is to add a shelf into the lower part of the legs. I've been collecting up these old beer boxes from all over the shop and wanted them to fit under the bench for storing all the nonsense you keep in a greenhouse. The last few bits of the bed frame were used and the bed base planks were cut to size and used there as well.
Job done. Switch on the kettle, turn on Radio 4 and away you go.