Bama Rambler is right. Test the salt level first and then adjust from that number. You might be shocked to find out there actually is already some salt in there. The salt strips are reasonably accurate and can be bought for about $10 at any pool store. (I can even find them sometimes at Menards.)
Even if you think you know how much salt you need, add a little less. You can always add more salt, but removing salt is a pain in the backside. (Trust me, I just recently had to do it when changing SWGs.) Since salt dissolves pretty quickly, you'll be able to retest in a 24-48 hours. Since the SWG isn't being installed until Saturday, you have time. (You're right to add it in advance of the SWG, though.)
When you do add it, then just dump it in and brush around. There's a thread somewhere about having the kids disperse it around with their hands. I haven't tried it but it sounds VERY cool.