Rate of tax under RCM depends on item you are purchasing. For example, if you paid rent then RCM will be at 18%, for GTA services it will be 5%.
You need to pay GST under CGST and SGST, like a regular payment. At the time of payment it is not required to disclose whether payment is under RCM or a normal payment.
RCM details are required to be furnished at the time of filing returns.