Hello,
What is recommended:
Recommended Sequence of Configuration Tasks - Electronic Account Statement (FI-BL) - SAP Library
Take a look on SAP help.
SAP IDES document is also available for SAP FI-BL
You have to do the customizing (analyse bank file an set up posting rules)
You make import of the bank file.
In accordance to the posting rules system makes posting, as an instance for incoming customer payments like:
DE: Bank (GL account)
CE: Customer account (sub ledger)
after this posting reconciliation of customer account has been done.
all the best Erwin