Step 1: Draw a line parallel to the dry adiabatic lapse rate starting from the temperature that is 50 mb above the surface.
Step 2: Draw a line parallel to the mixing ratio lines starting from the dew-point that is 50 mb above the surface.The intersection of these lines is the LCL
Step 3: Draw a line from the LCL to right parallel with the x-axis. Read off the LCL height