I put water in my new tank first, to make sure it was tight. I added the Instant Ocean. I ran the filter and heater until the salt had completely dissolved and the temp was around 70 degrees.
The Southdown was added in batches, each pre-rinsed in a bucket. This consisted of running warm water into the sand, while stirring it up with my hand. This would turn the water cloudy, and I would pour off the excess and repeat. This would also weigh the sand, so it would sink quickly in the tank. I rinsed until the runoff was partially clear, not totally clear.
It was tough to tell how level it was landing in the bottom of the tank, but I had a good idea how much was in there. I did not rinse the 10lb LS bag, which went on top, after I had around 2.5 inches of Southdown in the bottom. My tank was cloudy, but not a heavy thick mixture.
I ran my Marineland Bio-wheel and heater for 2-3 days. The tank cleared and it now looks sweet.
Drakken is, IMO, correct. It is a pain to drain the tank, but if you do, and do a partial sand rinse, you will, IMO, put yourself back on track. Best luck, maxpain.