The form is basically the same as the tax return, just in advance. They ask what your current/expected income is, they ask you to attach a payslip, current rent, estimate of rental and work expenses, add it all up and estimate your tax refund for the year, then the ATO contacts your employer and asks them to give you this in 12, 52nd or 26th incremenents depending on your pay period.
You can ask your accountant to help with the forms, though they may charge a (tax deductable) fee. Its worthwhile using an accountant if you dont feel comfortable I reckon, at least for the first time. They can work out how much you will be getting back, and perhaps underestimate your expenses a bit so you dont need to pay any tax at the end of the year.