Answer by taking a quote from the following article:

Seward was born in on May 16, 1801, in the small community of Florida, New York, in Orange County. He was the fourth son of Samuel Sweezy Seward and his wife Mary (Jennings) Seward. Samuel Seward was a wealthy landowner and slaveholder in New York State; slavery was not fully abolished in the state until 1827. Florida was located some 60 miles (97 km) north of New York City, west of the Hudson River, and was a small rural village of perhaps a dozen homes.

When the war started Seward turned his attention to making sure that foreign powers did not interfere in the conflict. When in April 1861, the Confederacy announced that it would authorize privateers, Seward sent word to the American representatives abroad that the U.S. would become party to the Paris Declaration Respecting Maritime Law of 1856, outlawing such vessels, but Britain required that, if the U.S. were to become a party, the ratification would not require action to be taken against Confederate vessels.  The Palmerston government considered recognizing the Confederacy as an independent nation. Seward was willing to wage war against Britain if it did, and drafted a strong letter for the American Minister in London, Charles Francis Adams, to read to the Foreign Secretary, Lord Russell. Seward submitted it to Lincoln, who, realizing that the Union was in no position to battle both the South and Britain, toned it down considerably, and made it merely a memorandum for Adams's guidance.  In May 1861, Britain and France declared the South to be belligerents by international law, and their ships were entitled to the same rights as U.S.-flagged vessels to remain 24 hours in neutral ports. Nevertheless, Seward was pleased that both nations would not meet with Confederate commissioners or recognize the South as a nation. Britain did not challenge the Union blockade of Confederate ports, and Seward wrote that if Britain continued to avoid interfering in the war, he would not be overly sensitive to what wording they used to describe their policies.  In November 1861, the USS San Jacinto, commanded by Union Captain Charles Wilkes, intercepted the British mail ship RMS Trent and removed two Confederate diplomats, James Mason and John Slidell. They were held in Boston amid jubilation in the North and outrage in Britain. The British minister in Washington, Lord Lyons, demanded their release, as the U.S. had no right to stop a British-flagged ship traveling between neutral ports. The British drew up war plans to attack New York and sent reinforcements to Canada. Seward worked to defuse the situation. He persuaded Lyons to postpone delivering an ultimatum, and told Lincoln that the prisoners would have to be released. Lincoln did let them go, reluctantly, on technical grounds. Relations between the U.S. and Britain soon improved; in April 1862, Seward and Lyons signed a treaty they had negotiated allowing each nation to inspect the other's ships for contraband slaves. In November 1862, with America's image in Britain improved by the issuance of the preliminary Emancipation Proclamation, the British cabinet decided against recognition of the Confederacy as a nation.  Confederate agents in Britain had arranged for the construction of Confederate ships, most notably the CSS Alabama, which ravaged Union shipping after her construction in 1862. With two more such vessels under construction the following year, supposedly for French interests, Seward pressed Palmerston not to allow them to leave port, and, nearly complete, they were seized by British officials in October 1863.

How did he make sure they didn't interfere?
Seward sent word to the American representatives abroad that the U.S. would become party to the Paris Declaration Respecting Maritime Law of 1856,