Wash it in 5 gallon buckets. Fill the bucket about 1/3rd to 1/2 with sand, then turn your hose on and push it through the sand. Let the water carry all tiny particulates and grains that want to float up and over the top of the bucket. Continue doing this until the water in the bucket is completely clear.
Fill the tank about 1/2 way and get your heaters working. Use a cup or a funnel to get the sand into the water...don't just splash it in. You could also lower the entire 5 gallon bucket down into the tank and slowly tip it over on its side, then slide the sand out with your hand.
Once all your sand is in the tank, pile it how you'd like and add decorations. Once everything is settled, you can probably turn on your filtration. That'll depend on whether your water in your tank is full of sand or not. If there's too much sand in the water column, simply let it sit for a few hours and see if it clears up. You may have to do a few water changes to take out the majority of the floating stuff. As soon as you think you're not going to tear up your impellers with sand, get your filters working on polishing the water. Then, begin your cycling of the tank.