Check out this video to learn a technique for attaching soft clay to soft clay. No scoring or slip is needed. By just applying firm pressure to get a good strong joint. The small clay coil is used to reinforce the joints, and the blending is to make sure both parts are joint securely. Finally give a little pull to the new piece of clay that was attached, to test how good the joints are. If it comes loose than you are not applying enough pressure in joining the two pieces together. Finally, if the clay is all the same softness, you don't have to wrap the whole thing in plastic to equalize the moisture of the clay to ensure an even shrinkage rate.