I have done the same thing you are doing just about 2 months ago. Its very simple. Take all of the fish out using a fish net, get a ziplock bag and fill it with tank water. Put the fish in their own individual bags and put all the bags in a bowl so they all stand up straight, to prevent the bags from leaking. Take out all of the decor, plants, etc. At this point you should only have water, sand, and equipment in the tank. Now start doing an 100% water change. turn off the heater and start draining the water. Once the water is drained to just ABOVE the filter intake, turn off the filter and continue draining the tank. Once you have as much water as you can out, its time to remove the substrate. get a small unused hand shovel, unused dust pan, or just your hands, and put all the gravel into large ziplock bags if you want to keep it. if you dont, put it all in the trash. Now you should have an empty tank. Its time to put the sand in there. If the sand is THOROUGHLY cleaned, dump it all in the tank, fill it up with CONDITIONED water. Its important all the water is conditioned before you put it in the tank, or the bacteria in your filter will die. After the tank is full of water, turn on the filter, put in decor, plants, ect, then the fish. Youre Done! If the sand isnt clean, fill a bucket half way up with sand, shove a hose in it and turn the water on half pressure. Now its a waiting game. Keep the water running and over flowing until you can clearly see the sand in the bottom of the bucket. The sand is now clean and do the steps afterwards.
Good Luck!