Each and every month proper entries has to be passed in the Books of Accounts / Accounting Software maintained, for Adjustment of Input Credit against Out put liability and statement has to be maintained for Tax Invoices which has been not disclosed in GSTR 2A / 2B.
Further at the end of year you have to prepare a statement as attached herewith. Various methods will be followed in the industry for reconcile the ITC. I have attached which I have maintained.
Closing balances as per statement has to be matched with Books of Accounts and Electronic Credit Ledger in GST Portal.