Health Savings Accounts

Hi,
I have many clients that have HSAs but don't fully fund them through payroll deduction so they make a prior year HSA contribution before 4/15 and then deduct that as an above the line deduction on their tax return. If you could create a flow chart explaining HSA contributions and ability to fund through payroll (already deducted on w2) and/or outside of payroll (deduct on return) that would be greatly appreciated!