Assuming that you are aware of RCM mechanism and sections regarding RCM I am discussing only disclosures to be made in GSTR 1 and GSTR 3B
First GSTR 3B
you have declare your liability first in 3.1(d) row. The details to be declared in that row are value of supplies received from supplier of goods or services and tax amount applicable to the supplies.
After declaring liability, in Table 4(A)(3) you have to claim input related to that RCM liability.All the liability you have declared in Table 3 you have to discharge the same through cash ledger at the time of filing of GSTR 3B.
Disclosures in GSTR 1
At the time of filing GSTR 1 there will be only disclosures effect in document issued table in GSTR 1.
As per GST rules when a payment is made to RCM transaction we need to raise a payment voucher and an Invoice relating to that supply and we have to disclose the starting number, ending number, no.of cancelled docs in related rows in Documents issued table.