Sir please help with basics of cash flow

Sir could u please explain me the basics of cash flow.I know its three part cash from operating activity,then investing,then financing.

In operating we add back all dep,amortisation,do we deduct gain and add back losses if yes they why do we do that.why do we add decrease in receivable,inventories and add in trade payable.
In investing activity we reduce everything like what ever we purchased(as cash is going out),and add what ever we sold(cash is coming in)
.
Same for financing activity if we have paid for anything like repayment of loan we reduce n if we get we add, am I right.

Its mainly the operating activity creating confusion. So could you please clarify as I do not want to spend long on this topic.

Many thanks.