Don't ever be sorry for asking questions
so, you have your tank, skimmer,lights, hydrometer - take that back and buy a refractometer
, you have a wave machine? what make and flow? You will require a basic turnover flow of x 20 so approx 7000 litres per hour. Make sure you get at least three powerheads for a tank that size unless you are going for vortechs.
Sand - what sand is it?
Lights - what lights are they, t8, T5 etc and how long is it since the tubes were changed? What tubes are they?
Do you intend keeping corals?
What test kits have you bought - most of use salifert, you will need, ammonia, nitrite, nitrate, phosphate to start with.
Water - you will need RO water, It is a royal pain to mix 350 litres of the salty stuff - i cheated when setting up my 400 litre and bought it from lfs - BUT, i knew what SG the water was sold at and the TDS reading - buyer beware, lfs have been known to sell water that is not much better than tap water!!! This gets you off to a bad start!!
So as soon as we have the answers to the above we can get you started with you tank .........woo hoo
Have everything in place first - you have to learn patience very quickly here on the salty side. But in answer to your question, as soon as you have the salty stuff in your tank, it has reached the correct temp and SG and has lots of lovely flow through it for 24 hours you can add your live rock first and then the sand.
Seffie x