Assuming you're eligible for SMP through your employer you just need to provide the MAT B1 form and they sort the rest. For maternity allowance it's a longer process with more forms.
Employer sent forms to fill in and I provided a Matb1 form at about 24 weeks. It was just information like due date, whether I intend to return etc.
employer gave me the forms to fill out and i handed it back to them along with my mat b1 and they deal with the rest x