Addition is a natural extension of counting. To start, you simply count one set, then continue counting with the second set.
Eventually, it will be helpful if the child memorizes the addition table for numbers from 0 to 9.
Addition Worksheets: With Pictures
Arranged from easiest to hardest
Addition Worksheets: Just Problems, No Pictures
Arranged from easiest to hardest
Addition Flashcards: Shown with one number above the other
Addition Flashcards: Shown with one number next to the other