Most treatment equipment is installed around its water connections, and the drain gets whatever space is left. That is backward. A drain that backs up, siphons, or spills at the floor can damage a cabinet or a basement slab, and it is often the last thing anyone checks. Treat it as its own job, with a route, parts, a build, and a test.
Plan the route before anything is cut
Find the drain point you will use, usually a standpipe, a utility sink, or a floor drain, and confirm it is allowed by your local plumbing rules. Measure the run from the unit's drain fitting to that point, and note the height of both. Ask the installer which connection type they plan to use and whether your local rules call for an air gap or air break. Get that answer before you buy tubing.
Gather the right parts
Use the tubing type and size the unit's manual specifies, not whatever is in the hardware aisle. Add the fittings and clamps the route needs, and an air gap fitting if your rules require one. Keep the packaging or the list so you can match parts later if a joint needs redoing.
Build the line to the manual's height and slope
Follow the manual's rule for how high the drain can start and how far it may run before it reaches the drain point. Keep the line from dipping below the unit's drain connection where water can sit and siphon back. Support the tubing so it does not sag, and avoid kinks at every bend. Cut each end square, push it fully into its fitting, and clamp it where the design calls for a clamp.
Test it while you can still see it
Run the unit through a regeneration or backwash cycle while someone watches the discharge end and every joint. Look for drips, air spitting at the air gap, and any water that rises back toward the unit. If the drain shares a sink or standpipe with another fixture, run that fixture during the test too, because a shared drain can back up only when both are in use.
Record the route before the walls close
Take photos of the finished run, the fittings, and the drain point while they are accessible. Note the route on paper or on your phone. Someone troubleshooting this line in a year will need to know where it goes.
Know where to stop
If the route needs a new drain opening, a cut into a drain stack, or a change to a vent, stop and hand that part to a licensed plumber. Ask your local building or plumbing office who is allowed to make those connections and whether a permit applies. You can still do the parts that are yours, such as measuring, fitting the tubing, and recording the result, while the plumber handles the connection to the drain system.