How Can You Install a Recessed Niche in an Existing Wall?

The first step is to choose a spot, trace a template, and cut an opening. The opening should be plumb and parallel with the floor, and be between studs.
Next, cut blocking from 2x4 lumber that fits snugly between vertical studs (see photo below), so that it sits flush with the opening's edges on both sides. This will provide a surface for the niche to attach to, as well as keep the wall structure from being compromised.
When cutting blocking, make sure it's about a half inch longer than the niche, so that it can be screwed in place securely. Then, install the niche into the opening.

A shower nook is a recessed storage space in the wall of a bathroom or shower. These recessed shelves can accommodate a wide variety of bathing accessories, such as soaps, shampoos, conditioners and other toiletries.

These recessed niches are available in two different types, a tiled shower niche and a finished recessed niche. A tiled niche is a waterproof, permanent recessed shelf that's installed in the wall.
Finished niches come in many different varieties, including those that can be retrofitted into a existing tiled wall. They are also available in a number of different materials, such as plastic and steel.

These recessed niches are not the cheapest option, but they are durable and can last for years. However, they should be installed by professionals due to the risk of water leaking into the wall cavity if not done correctly.