Draw a horizontal line on a sheet of paper. Mark the center of the line as the zero value. Mark positive integers on the right side of the line and negative integers on the left side starting at +1 and -1. This creates a number line.
Subtract two negative integers such as -5 and -3. Subtracting two negative integers has the same result as adding the positive value of the number you are subtracting, since the two negative signs cancel each other out. Therefore, the result of -5-(-3) is -2. Start at -5 value on the number line and move three integers to the right to obtain the results.
Add two negative integers such as -4 and -8. Adding two negative integers is similar to adding two positive integers. The result simply obtains a negative value. Start at the -4 value and move eight integers to the left. The result of -4+(-8) is -12.