Hi Vero. Here is a great post with directions on how to stack pots and have the flowers cascade out... https://www.instructables.com/id/Tipsy-Pots-Tower-Planter/
Any plants that say " creeping" or " climbing " in their name will naturally grow to spill over your pot edges creating the effect I think you're trying to achieve. Happy planting :)
Here are some ideas for you.
My two favorite plants for this are the sweet potato plant and creeping jenny. You can find out more about that here: https://thehoneycombhome.com/container-gardening-recipes/
Hanging petunias or mini petunias love to hang down. Any pots on a stump, a pedastal, or even bricks or corner of the porch will support your pot or the flowers. You are on the path of a great idea! If it were me, I would look in the nurseries or department stores that sell the and see what they have done to "advertise" them and see what you like and what will be pretty in your yard.