ITC wrongly claimed in gstr 3b

I have wrongly claimed excess chat/sgst in gstr 3b for the month of apr'19 of RS. 2000 each and used the same.
now in June month I realized that it was not to be taken.
now I have 3 questions,
1 - how much interest has to be paid and for what number of days

2 - and how should I show this in return? should I adjust it in gstr 3b return for June month or should I show it under form drc 03

3 - for making June month gst payment do I have to prepare separate challan for June month and this earlier gst payment or can I make one single challan for it

Thanks in advance