Get some buckets fill them with tank water. Put all your fish in them. And move them into a dark room to keep them calm.
Then drain all the water from it remove all decor (obviously) and filter.
Make sure you wash the sand thoroughly, if you put the sand in a bucket and run a hose pipe into it to fill the bucket and leave it running so it overflows and swish the sand around until there are very little- no small particles floating in it (if you are in the UK and cant use a hose pipe use the bath).
Then put the sand in. Fill half way with dechlorinated tempreture matched water, net the fish back in pour the water from the buckets back in, sort out your scape put the filter back in. fill the tank up again with temp matched dechlorinated water and hey presto.
It will cloud up a fair bit no matter how much you cleaned the sand, but the more you cleaned it, the quicker it will clear.
Oh and do water tests everyday for a week just incase you go into a mini cycle.
Hope that helps and if I've missed anything someone will say so.
Edit: Reduce the amount of food you give them a week before and a week after to minimise ammonia build up when in the buckets and reduces the chance of a mini cycle.
P.s when emptying the tank clean the gravel very well with your gravel vac