Reverse charge mechanism

Dear Sir,

Please help me understand the reverse charge mechanism for following case: I have received goods and have an invoice from transport operator where he has mentioned that gst under reverse charge has to be paid by me. Date of Invoice: 15-07-17. Date of Payment: 18-07-17. 

So I will issue an invoice on 18-07 under reverse charge charging 5% gst and where do i include this in next return ? - in b2b sales with my own gstin in place of gstin of receipient and reverse charge column 6 as yes?

Now when do I have to make the payment for the tax under this rcm - normally on 20th of next month? or date of invoice i.e. 18th July itself? as it says you cant use credit ledger but only cash ledger. what does this mean?

Thanks