The acid is usually mixed with about ten parts water. I use it in a Windex type squirt bottle. You'll need to do a little scraping, the acid works real well removing the mortar color, but not the sandy lumps of mortar. The acid will kill plants, irritate your soft spots, sting your eyes like smoke, rust your steel in a minute. Usually it's best to wait at least 28 days to squirt the new work with the acid. If it's outside there usually isn't much problem with fumes. When your done, wash it real well with a hose. There's been several jobs where the acid was used full strength and allowed to dry without any problems showing right away. An old mason would suggest you wet the work with water first though. You really can't mess it up too much unless the bricks have a finish that also washes off. Start will a spare brick and see what happens. Most any type of stiff fiber brush will do the job, you know, a scrub brush.