Bundle 100 mortgages at average sale price $100,000. 100 * 100000 = $10 million. Double that to count the average interest on mortgages. 2 * $10 million = $20 million. Tranche it up so the top 20 creditworthy mortgages pay the investors first. That top tranche is then arbitrarily valued by a trader, say at $40 million for that tranche alone (face value was 20 * 100,000 * 2 = $4 million).

The other 80 mortgages go in lower tranches and are marked up arbitrarily. RBS traders sell them off. Once they are sold, RBS has no further liability regarding them. 

Even if RBS can only sell the top tranche, it makes money.

The buyer can pledge the top tranche Mortgage-Backed Security at the Fed, or with private money markets. The Fed or some private money dealer will issue $40 million and hold the MBS as collateral. The loan can be rolled over. Other MBS can be used to pay back the loan, and the Fed can declare it as an asset and issue money against it. A private Eurodollar dealer can create Eurodollars to pay for the MBS, and the created Eurodollars can be used to pay taxes. Note that the Treasury is not required to deposit tax receipts at the Fed. If Citibank creates dollars and pays taxes with those dollars, the Fed can put the deposit in Citibank and no Fed reserves are required.