Vacuum the entire area you want to stain. You must remove all dirt and debris off the concrete before staining can begin. Any buildup can prevent the stain from properly interacting with the concrete and produce a less-than-stellar final product.
Inspect the concrete for cracks and holes. Leave hairline cracks and pinholes alone, but fill in larger cracks and holes with concrete glue. Let the concrete glue harden, then sand down with a sheet of sandpaper to smooth out the surface.
Scrape off any stains on the concrete with a wire brush. Use a mineral and rust-deposit cleaner to help remove these types of notoriously difficult stains. Spray directly on the stains and use the wire brush to scrub off. Dry with paper towels.
Vacuum the floor again to remove any remaining dirt and debris. This picks up any debris you created during the sanding process and the general cleaning process.
Lay down plastic covering, secured by painters tape, to protect any furniture, fixtures or other items that you need to protect from the acid.
Put on a pair of safety goggles, face mask, and acid-resistant gloves, pants and shoes, to protect you from the dangerous acid you must use. Also, open all windows and doors, if staining indoors.
Apply the acid stain with an all-plastic garden sprayer. Pump the sprayer to create pressure and trigger the nozzle to release the acid stain. Point the nozzle directly at the areas you want stained. Use caution while spraying. Even though you have protective plastic covering important items around the work area, the acid can still eat through the protective covering.
Distribute the acid stain with a clean rag. Rub the stain across the concrete floor with the rag. This creates a textured appearance not possible with smoother application devices such as paint brushes and paint rollers.
Allow the stain to dry and set into the concrete for 24 hours. Use a wet/dry vacuum cleaner to vacuum any remaining acid-stain liquid from the floor.