I recommend doing it this way:

1.If you dont have one buy a 40g trash can from lowes or home depot
2.do a 10g water change the day before and put that water in the can with a power head and heater
3. syphon 10 - 20g water from the cube into the can
4. remove all rock, coral, and livestock from the cube and put them in the can
5.If you use new sand wash it thouroughly and let it dry somewhat before you add it to the cube
6. If you use used sand from someone elses tank and it stinks like rotten eggs then toss it
7. I recommend using arag-alive and seed it with a cup or two of sand from another tank
8. Add the the sand to the cube...it still should have at least 10g of water left in it. Let the sand settle. If needed syphon some water from the can to the cube
9. Turn on heaters and power heads and let everthing stablize
10. Two choices here: wait a day or two and test parameters before adding livestock and the rest of the water, or add everything to the tank at once after the sand settles.
11. you will probably have about 20g of leftover water

HTH