For TDS on salary you have to fulfill 3 conditions 1) Threshold limit is 250000/- , TDS deducted on average . 2) Employer and Employee relation 3) TDS must be deducted from Salary .
You have to first calculate tax on such salary, then divide such tax with gross salary. This will be your % of tds of that particular salaried employee.
Then you can deduct such % while paying monthly salary.