Find a corner in which to install the safe. Having both the back and one side against a wall gives any thieves only two sides, and a limited area, to attack.
Place a thin foam panel under the safe when setting it into the final position. This will help keep moisture wicked up from the concrete floor from entering the bottom of the safe. If the basement walls are bare concrete then place foam panels between the walls and safe.
Drill holes with concrete bits into the basement floor matching the security holes on the floor of the safe. Use an expanding anchor with a bolt head to secure the safe to the floor.
If the safe has security holes in the back wall, then repeat the process to anchor the back of the safe to the wall. Leave enough room between the back exterior of the safe and the wall to allow an electrical cord for the dehumidifier to run in the safe.