Before you do anything, make sure 100% that the water you got from the LFS does not contain any kind of medication such as copper (some LFS' run copper in all their fish-only tanks) if it does you could effectively ruin the sand and LR permanently.
If the LR is already fully cured then place it in the tank and see what kind of 'cycle' you get (sometimes, with fully cured LR, there may not be a noticeable cycle at all).
Measure the A,N,N levels over the first week or two, just as you would with a FW tank cycle.
Do not put any animal in there until you have created a safe environment for it to live. Remember, Ammonia and Nitrite are much more dangerous in Saltwater because the PH is so much higher than FW.
This should be your mantra:
Nothing good happens quickly in a Reef Tank.