I recently set up my new 55 gallon with sand and I much prefer it.
All of the kid's play sand I found said 'not for use in aquarium'. You have to be careful to get sand that is inert, else your Ph is going to skyrocket through the roof. Try and get hold of silica sand, also known as 'pool filter sand'.
Take your fish out of the tank and put them in a bucket with a small heater and air stone (if you can). Remove all the water and decorations and gravel from the tank.
Pour some sand into a bucket til it's about 1/3 full. Hook up a garden hose and fill the bucket with water. Swish the sand around with your hand lots. Empty the water out and then put the hose back in, let the water flow through the sand some more, continuously stirring the sand with your hand. Do this until the water that overflows the bucket is running clear. Empty the water out, put the sand into the aquarium. Repeat until all the sand is washed and in the aquarium. This can take a while, but the more you swill it around the quicker it goes.
When you refill your aquarium, the water will still be a little cloudy but don't worry, it will settle.
Oh, and I just through of something. Since you're switching from an already established aquarium, you should probably keep your filter running. If you have a spare aquarium set up you should run it on that while you're working.