Eliminate all unwanted plants from the yard by digging up shrubs and choking off other ground plants and dormant seeds and spores. After digging up and pulling any undesirable tall plants from the yard, cover the entire yard space with opaque plastic sheeting to block sunlight and moisture.
Cut slits to form an "X" in the plastic to fit and accommodate any plants you wish to retain for your gravel backyard. Tape back the slits from the hole to form a snug collar around the base of the plants to save.
Dump wheelbarrows of gravel into the backyard and rake it into a reasonably level 3-inch layer. Continue dumping and raking until the plastic sheeting is completely buried in gravel.