First tweak the settings of the Brush Tool to create an eyelash-like brush. Select the Brush, and in the Options Bar, click the Airbrush Icon to activate Flow. Click the button to open the Brush panel and click on the word Transfer. Set the Opacity Jitter control to Pen Pressure, then click on Shape Dynamics. Set the Size Jitter Control to Fade, and set the Size Jitter to 80% and the Control to 80. (If an 80% Jitter looks too rough, reduce the size until the brush matches the lashes you are working on.) On this image, a 4-pixel brush matched the size of the actual lashes. Make a new, blank layer, sample a color from an existing lash, and draw in new lashes.