What is the proper way to launch a softbottom inflatable using a sling? What is the proper sling to use how do you attach it to the boat?

the best way is to have 2 eye hooks put into your transom. most are marine plywood with neoprene coating so you can use an ordinary wood drill bit. you can pick up a tri-harness similar to jet ski lifting harness and connect two to the eye hooks and use your forward "D" ring on the bow for the forward connection. I recommend you put a seperate "launch" "D" ring on the inside of the bow tube if you can so the lifting harness does not rub on the outer tube and create maring.