Just add the LS on top of the sand you have now. Let the bacteria spread by themselves. Stirring your old sand bed will probably kick up a lot of bad things that may have accumulated on it. If you don't want your tank to cloud take some tubing or pvc piping with a funnal and lower it right where you want the sand and just pour it in letting it go down the tube. It's what I did...worked great!
HTH
SaltyShark