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Reattach the drain plug to the base of the tank. Pour new pool silica sand into the tank. Fill the tank with water up to the level of the sand. Lubricate the O-ring on the valve with a multipurpose lubricant. Reattach the multiport valve. Screw the collar and pipes back in place.
Before adding any new sand to the tank, you'll need to reattach the drain plug. Screw it back in place at the base of the tank, making sure to tighten it in order to prevent leaks. Position one corner of a bag of pool-grade silica filter sand over the mouth of the tank. Make a small cut in the corner of the bag to allow sand to slowly fall into the tank. Work slowly and carefully to prevent any sand from spilling. Repeat until you have added the amount of sand needed for your filter. If you are worried about damaging the components on the inside of the tank, add a little bit of water to the bottom of the tank before pouring in the new sand. This will cushion the sand as it is added but may misalign the standpipe. As you add sand, make sure the standpipe in the center of the tank stays right in the middle and at the right height. It will need to reattach to the base of the multiport valve when you are finished, so you can line the two up to check if you're unsure. The amount of sand needed will vary between pool filters. Check for stickers along the side of the tank or in your tank's manual to find out how much your tank needs to operate. Use a garden hose to add water to the tank until it just begins to cover the sand. This will give the tank enough water to clean the sand and get the pump functional before you start filtering your pool water. Locate the rubber "O-ring" around the top of the multiport valve where it will form a seal with the mouth of the tank. Apply a small amount of lube to your finger and rub it around the O-ring. This will make reattaching the valve to the tank easier, as well as conditioning the rubber seal. If the O-ring is damaged, you might need to replace it rather than just lubricating it. These should be available from hardware and pool supply stores in the size you need. Remove the tape or plug from the top of the standpipe and position the multiport valve over it. Carefully connect the opening in the valve to the top of the pipe, and firmly push the valve into the top of the tank. Wiggle it around slightly as you push it into place to make sure it is secure. Position the collar around the edge of the multiport valve and use the two bolts to tighten it into place. As you do so, alternate between bolts to ensure the pressure is equally distributed around the collar. Reattach the pipes and screw the unions on as tightly as you can to prevent leaks.