How to add to a curtain that is too short?

Sew, velcro or stich witchery additional fabric to the bottom (or top, if you are so inclined.) Here are a couple links to help you out!

http://homeguides.sfgate.com/fix-curtains-short-27936.html

http://abeautifulmess.com/2015/12/lengthen-your-curtains-without-sewing.html

If you've run out of fabric , you can add sheers to the bottom .

Think adding material on TOP, not bottom

I would add a little fabric to the top and the same fabric to the bottom.

Add one or more contrasting stripes of fabric to the bottom.... this style is real trendy right now... http://www.brylanehome.com/decor/Triad-Faux-Silk-Grommet-Curtain.aspx?PfId=117231&DeptId=15286&ProductTypeId=1&ppos=28&Splt=0&StyleNo=1473

Measure up 6" from bottom of Curtain then cut along that line. Insert contrast fabric then add back the bottom of the curtain.

Lower it so that the hem reaches the floor, and fill in the top with another rod and a pretty valance.

Find a fabric that matches or is a nice blend/accent. Add it to the bottom like trim. Mke tiebacks out of this same fabric or add tabs to the top.

get a curtain stretcher