Inspect the job carefully. With a correct combination of electrode size and current setting, the area of the weld should be a complete fusion of the electrode metal with the metals being joined. Slag forming on the surface should be chipped away with a chipping hammer or pick. ALWAYS wear your safety goggles when chipping away slag.
NOTE: If the resultant weld looks irregular or messy, or shows signs of porosity or slag contamination, you have almost certainly failed to achieve the correct combination, (or dirt /oil is present on workpiece). Do not worry as practice will soon cure this