Stucco is easy to cut through but can be somewhat fragile. Take a couple minutes to inspect the area where you will be cutting.  If you notice any cracks or deterioration in the stucco you should realize that the vibrations caused by cutting will probably cause further damage. You will need a circular saw with a diamond blade and a cold chisel.  Once you have drawn the outline of your ledger board on the stucco you can cut through the stucco and the metal screed it uses to grip to the house. Be careful not to cut too deep and concentrate on cutting a straight clean line. 

Cutting away stucco siding will be very messy and will create a lot of falling debris and dust.  Be sure to wear a mask or a ventilator to prevent breathing in too much dust.  When the house rim is exposed you can attach the ledger board as described below.  Seal coat the exposed edges of the stucco to prevent water infiltration