I don't know where you are, but in NA both Home Depot and Lowe's carry Quikrete Play Sand. I have seen two shades, one a buff and one dark grey. The dark grey is best if you can find it. My HD only carries the grey, so it may depend upon stores. I have heard that Lowe's also carry this Quikrete Play Sand. One bag (they are 50 pounds/25 kg) is all you need.
The sand will make quite a mess, so I would drain the tank completely (keep the filter wet in a bucket of tank water for as long as this takes as the bacteria will remain alive if wet). Rinse the sand, add it to the tank in roughly a 2 inch overall depth. If you have wood, etc, I would place it on the sand before adding any water, it avoids more disturbance. When you add fresh water it will stir the sand up again, but it will settle out. Sometimes I drain it again, and then slowly pour new fresh water into a bowl on the bottom to avoid excess disturbance. Use water conditioner when you are finished and don't intend draining the water and refilling again. Then you can hook up the filter. Add the bio supplement last.
You do not need to use old water from another tank, this really has no benefit at all.
