The bottom of a walk-in shower consists of a tray with a drain. During installation, the sealing process is fairly simple. A leaky shower can be a result of a worn-out seal around the drain. Resealing an already installed shower tray and drain can present some challenges.
Check plumbing codes and following the installation instructions that came with the drain, up to the point of applying a sealant. Loosen the compression nut in the drain body and apply a bead of waterproof silicone around the underside of the drain top. Push the drain body firmly down into the drain hole in the shower tray. Continue the installation according to your instruction manual. Do not forget to tighten the compression nut and remove any silicone that squeezed out.
Repair of an installed drain is a bit of a challenge but it can be done. Shower trays are made of different materials but the product used to seal the drain will always be waterproof silicone that is made for bathroom fixtures. You need to remove the strainer and loosen the compression nut. Use a putty knife to carefully scrape out the old silicone. Generously fill the underside of the drain top with silicone and tighten the compression nut. Remove excess silicone from around the edges. Wait 24 hours, replace the strainer. Wait another 24 hours before using the shower.
Using a high quality silicone should reduce the chances of having to repair it again in the near future. Purchase the silicone in a squeeze tube, not a caulking tube. A squeeze tube is easier to work with in tight quarters and the quality is generally better. Make sure the silicone in waterproof and labelled for plumbing.