how to claim ITC?

It has to be reclaimed through GSTR-2. Currently the Form GSTR-2 is not available. Therefore, you need to consider as eligible credit in table 4 of Form GSTR-3B.

The amout revsersed due to non payment of consideration is eligible credit as and when the consideration is paid. The amout of ITC shall be maintained in separate ledger. The same cannot be charges to P&L.