Drain the pool if it is not already empty. Turn off any equipment, and then close the skimmer valve so that the main drain is open, and on the automatic panel select the option to drain the pool. The description may differ from company to company, but all should have a "drain" option available.
Set the drain to "off" on the automatic panel, or if the option is not provided, turn off the suction pump. Unscrew the drain cover with a screwdriver or suggested tool. Do not use anything electrical or requiring a plug when removing the cover or standing on the pool floor.
Check the filter to see if it needs cleaning (and if so remove it and rinse with clean water), then place the drain cover over the hole and carefully screw it in with the same tool used for removal. Follow any additional instructions in the manual.