You take the number on Line 38 of your 1040 and figure out what 7.5% of that income is. If you paid more than that amount in medical or dental expenses, you can deduct the amount over the 7.5% as a deduction.
Ex. Income on line 38 is $50,000. 7.5% is $3750. If your medical expenses are $5000 then you can deduct $1250.