Since an integer is any whole (not fractional) number on the number line, all you have to do is move to the left however many units you're subtracting by from the point given. Say the problem is 3 - 7. This means we start at 3, and move to the left 7 units, which lands us at -4. However, if we had a problem like 12 - (- 2), the two negatives would cancel out, meaning we will move to the right 2 units on the number line, or 12 + 2, which is 14. Some other examples are:
-12 - 4, (you should get -16)
3 - 8, ( you should get -5)
16 - (- 3), ( you should get 19)
I hope these tips helped you out! I know that when I first had to subtract integers, using a number line really helped me out. Eventually, it'll just be a natural thing. :D