Short answers, pulled from the story.
The Blockade of Germany, also called the Economic War, was an Allied operation to restrict Nazi Germany's access to minerals, fuel, metals, food, and textiles needed to sustain its war effort. It was carried out mainly through a naval blockade by Britain and France, supported by the bombing of economically important targets and by buying up war materials in neutral countries to prevent their sale to the Axis powers.
The Nazi-Soviet Pact of August 1939 opened a land corridor the naval blockade could not close. The Soviet Union accepted American goods at its Arctic and Black Sea ports and released equivalent volumes to Germany in the west. A major trade deal signed in February 1940 was valued at 640 million Reichsmarks, with Germany receiving Russian grain, oil, and raw materials in exchange for military technology including a Panzer III tank and the Messerschmitt Bf 109.
The Navicert, short for Navigational Certificate, was reintroduced two months into the war as a commercial passport for goods certified as non-contraband by a British ambassador in the country of shipment. Ships carrying a valid Navicert could pass Contraband Control patrols without delay, while ships without one faced lengthy inspections and possible seizure of their cargo.
Germany responded with its own counter-blockade using U-boats, surface raiders such as the Graf Spee, and magnetic mines dropped by seaplanes in British harbours and estuaries. By the end of 1939 Germany had sunk 249 ships by U-boat, air attack, and mines. Germany also negotiated barter deals with neutral and Soviet suppliers and developed ersatz industries to produce synthetic substitutes for blockaded materials.
The Ministry of Economic Warfare was established in the final weeks before war broke out by Frederick Leith-Ross, the British government's chief economics adviser. It employed over 400 staff including bankers, statisticians, and international law experts who sifted intelligence, tracked ship movements, compiled import quotas for neutral nations, and maintained the Statutory List of 278 companies and individuals known to trade with Germany.
Germany announced rationing of food, coal, textiles, and soap on the 24th of August 1939, a week before the invasion of Poland. Men were rationed to one overcoat, two suits, four shirts, and six pairs of socks for the duration. By mid-September 1939 journalist William Shirer noted the blockade had already cut Germany off from 50% of normal imports of nickel, cotton, tin, oil, and rubber, and that Christmas 1939 many Germans went cold due to coal shortages despite sufficient food still being available.