For both freestanding showers and those abutting your home's walls, you'll likely want to direct the runoff using a border that will prevent water from spilling off to the sides. If the outdoor shower is near your garden, you can direct the runoff to help irrigate the flowers and plants. For a shower with the back wall attached to the home, you'll want to direct runoff away from the home's foundation.
You can make a border 5 or so inches (12.7 centimeters) high on each side of the concrete floor. A popular option is a glass block border. You use the thinset as you did with the back wall of the shower, applying it to the right and left edges of the concrete floor, then place the glass blocks side by side, back to front, with thinset between them to make them adhere to each other. For more privacy, you can continue building the glass blocks vertically until you have glass block privacy walls on each side of the outdoor shower. You can then install a shower curtain rod between the glass block walls and hang a curtain for more privacy.