Create bare ivy vines on the walls by sketching them in with a medium round paintbrush using medium green paint. Look at real vines to guide you in making curvy lines with lots of smaller stems and tendrils winding off the main lines -- larger at the bottom and gradually tapering to simulate new growth at the tops.
Lay real ivy leaves upside down on a work table covered in old newspapers.
Apply a very sparing coat of paint to cover the back of each leaf, using a flat paintbrush, turning the leaf into a natural stamp. Paint only a few leaves at a time so you can use them before the paint dries.
Place one leaf, paint side down, against the wall at a place where you want a leaf – positioning the larger leaves near the bottom, and the tiny, new leaves at the vine tips, rubbing each leaf from the unpainted side with your fingertips or a soft cloth to transfer paint from the leaf to the wall. This will give you an imprint of veins and the leaf outline with little excess paint.
Continue stamping leaves on the wall until you have a pattern that pleases you. Don't worry about making each leaf perfect at this point. Just work on the overall design in one basic green color.
Paint over the stamped leaves where the print is indistinct or where you want more detail using small round brushes and a variety of green colors ranging from dark for mature leaves to a light yellow-green for new leaves. Similarly, go back over the original vines, adding subtle shades of green and brown color here and there to create shadows and highlights for a three-dimensional effect.
Add painted shadows beneath leaves for a more realistic appearance by mixing white and black in varying amounts to create lighter or deeper shades.
Finish the faux ivy walls with a compatible, clear acrylic sealer to protect them.