Subtraction, as you may already know, is the opposite of addition. Simply just take away the numbers that you have put on from your original number. Heres an example:

- 5-2=3

- 111110 101-
- 111011 110=
- 110111 110

Practice these till you can figure it out. It may not be as speedy to make out but over time im sure you will have a good course in it.