How does lady Macbeth get king Duncan's blood on her hands?

In Macbeth right after Macbeth kills king Duncan he starts freaking out and forgets to plant the daggers on the guards as planned. In result of this Lady MAcbeth takes the daggers and plants them on the guards but is missing the blood to plant on the guards. She goes to the now dead Duncan and takes blood from his corpse and puts it on the guards.