Where you install the outdoor shower depends on its function. If the shower is simply for keeping dirt out of the house before going inside, then installing the outdoor shower on the outside wall of the house fairly close to the entry is most practical. If the outdoor shower is for rinsing off before a dip in the pool, you'll want it near the perimeter of the pool. It would do no good to shower near the house then track dirt with your wet feet while walking across the yard. If you want to shower while gazing at your colorful garden, or you're fortunate enough to have vistas of valleys, mountains, a lake or the ocean, you'll want to position your outdoor shower to take advantage of the panorama.
The spot you choose should be level, with plenty of soft soil beneath (if you don't place the outdoor shower on soft soil, you may have to design a drainage system). Also consider whether you prefer your shower in the shade or in the sun. Most people enjoy the warmth of the sunshine along with the refreshing water spray. Plus, positioning your outdoor shower toward the sun will help dry the soil more quickly.