Simply start counting number and see, if you can count without making any mistake. start counting from 1 to 20 then 21 to 50. If you can count means you have good focus, you can focus on next number and keep on continuing the work (counting) without any disturbance.
this is a simple practice to try out focus strength.
Remember - emotions or thoughts derived from fear, greed, expectation, lust anger, jealous
will be the main reasons not to have focus and once you recognize this, it would be easier to have focus on your work.
Note : when you want to be focus or be in presence, you have to consider this activity as a sport and carry sportsmanship.
Any `thought or fear` coming in between your focus is like a storm.
Why storm? because.. similar like storm, it remains for some time, after that time period you can focus. Negative things are temporary.
More the spirituality, more good habits. Keep trying and practicing...