I have also found that using a piece of the plastic sign board helps in getting new soft comb into a frame.
With the bees off of one side of the comb lay it on the sign board, trim to size if needed, lay the frame around the comb (have rubber bands on both ends). Now tilt the sign board, comb and frame up and pull the rubber bands around it. The sign board just gives the new soft comb plenty of support, not like trying to pinch it between your fingers.