interest on delayed pmt undet GST

Int need to pay at 18% and pay in gst website by showing amount in int coloumn in challan

ok. Can he pay int.for all those delayed pmt, in one single challan? Also.pls inform how to report this PMT of interest, when paid, on the portal.

Yes, you can paid all interest payment in single challan.

While filing GSTR 3B, entered Interest amount in interest column as per your calculation

for GST late payment. In case it is determined that the taxpayer misstated output tax liability in the GST return, then interest at the rate of 24% would be applicable. In addition to the interest, penalty could also be levied on the taxpayer under GST for erroneous return filing, wilful misstatement or fraud.