Decide the size you want your planting bag - using a box like a cereal or shirt box will help you visualize the size and create the bag itself.
Once you've decided the size, you want to cut some landscaping fabric to "wrap" the box just like you would a gift leave an extra inch or so at the top to create a reinforced lip.
Start by folding over what will be your top edge and taping down the seam, you want the tape to go all the way to the top and seal the rough edge.
Then, leaving the top seam you just made open, wrap the landscaping fabric around the box sealing all of the seams with duct tape, make sure that you run the tape the entire length of the seams.
For the bottom you want to cover the entire length and width with tape to reinforce and add strength to your bag.
Once you have the bag constructed, slip it off of the box and flip it inside out so that the seams are all on the inside. Slip the box back inside and cover the entire back of the bag with duct tape.
This will add strength and make the back waterproof so that you don't damage your wall or door.
To make a hanging handle, punch holes into the top reinforced edge and run some rope or wire through and knot it off.
Fill the bag with lightweight potting mix and use a utility knife to cut small "X"s where you want your flowers.
Insert the flowers into the slits and you're all set. Keep the bag watered and watch it grow - I'm Mrs. Fixit and it's just that simple!