ILR New Application Service

Hi

Can you please help me understand the new service.
I am looking to apply for ILR in Jan2019, I have filled my online application but yet to pay. My employer has told me that they would give me the letter first week in JAn. So I will not have that leetr when I pay and upload the documents. Does this mean I have to wait until I have all the documents before I proceed further with the online application ? Or can I just pay and submit and then take the document with me when I go in person. How does this work.

Any help with be appreciated.

Many Thanks.
Ashr