Line the inside of your mailbox with your cloth. Do your best to not have any gaps or openings so that the full interior is well lined. This will prevent soil from leaking out of the mailbox over time.
Add potting soil so that it is loosely filled about half to one inch from the lip of the box.
Gently push the fabric down, into the soil so that it is no longer visible.
Gently add your plants, pushing into the soil and packing it around the root ball so that it holds firm. Once all plants are added water them in.
Secure to fence or wall if desired.
As time goes on and you can tell how the plants are going to fill out, you can add more planting. I wouldn't advise going over 4 plants so that all of them have ample root room.
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