Sit or lie down, close your eyes, do the relaxation exercise and clear your mind. Breathe deeply and slowly and focus on the breath entering and leaving your body. Feel it coming in and feel it going out. Focus your whole attention on your lungs and the breathing process. This simple action is enough to occupy your surface mind. Firmly push intruding thoughts away, as they begin, before they can gather strength and distract you.
Breath awareness occupies the surface mind and allows you to think on a much deeper level.
Then move on to Getting rid of surface thoughts.