I've done it three ways.
1. After racking wine into secondary, transfer lees into a container and put it in the refrigerator until I prepare the ingredients according to the recipe. Then pour the lees into the primary.
2. Prepare the water and other ingredients a couple days ahead to save time, then just pour it all into the primary containing the slurry.
3. After racking wine into secondary, pour the water, lemon juice, table sugar and other ingredients right into the primary containing the lees without any preparation and go from there.
Basically, when you transfer your wine to secondary you start the SP using the slurry instead of a yeast packet. You'll want to make sure do the transfer while your wine is still actively fermenting. I've also used a depleted slurry just for the color and added a EC-1118 packet. SP is so cheap to make, it's ideal for experimenting with. So far it's all made SP just fine.