I think some of this is unnecessarily complicated. I have changed substrates so many times.
Siphon tank water from near the surface into the container in which the fish will temporarily live. And, it is never a good idea to change out the substrate (entire) with the fish inthe tank. It is not only severely stressfulon the fish, you stand the very real chance of poisoning them from disturbing anaerobic areas of the substrate as one of our members did recently. Put some of the decor into the temporary fish container to ease the fish, and float some plants on top. Then remove everything from the tank and clean the tank; do not save the old water, it has no benefit but quite the opposite. Add the washed sand to the tank, about 1.5-2 inches uniform depth (this is a 20g tank); add the decor, plants. Fill with fresh water, use a conditioner only. When the heater and filter are working and everything looks OK, net the fish in. Make sure the parameters (GH, pH and temperature) are reasonably the same between fish container water and the new tank.
Washing the sand is easiest in a bucket. About five or six cups of sand, rinse five or six times, drain out the water as best you can, dump the sand into the tank. Once you have the decor and plants, it may be quite cloudy; I usually drain the tank down to the sand and carefully refill running the water into a large bowl or similar as this will overflow and avoid disturbing the sand. The dirt in the sand is just dirt.
