He read the incantation and pronounced it well enough that it worked.
Although Harry didn't know what Sectumsempra did, and didn't have explicit instructions how to cast it, he had seen it written out in the Half-Blood Prince's book.
There are at least two ways Harry could have managed to cast Sectumsempra without ever getting told how to pronounce it. Sectumsempra could have been easy enough to figure out how to pronounce by just seeing it written - certain words are easy to figure out how to pronounce without ever having heard them, and the same thing might apply to spells. In addition, he might not have pronounced or cast it perfectly, just well enough for it to have that effect on Draco.
If Harry was being graded on his casting of it, he may well not have gotten the top grade, he might have even failed. It had an effect, but likely not the exact effect that Sectumsempra has when cast perfectly.
Some spells don't seem to require special wand movements, so Sectumsempra might not.
Wingardium Leviosa requires a specific wand movement, but that doesn't seem to be the case for all spells, even more difficult ones. Some spells seem to be more 'point and cast', since specific wand movements aren't always shown being taught or used.
Lumos, for example doesn't seem to require a specific wand movement.
“Lumos,’ Harry muttered, and a light appeared at the end of his wand, almost dazzling him. He held it high over his head, and the pebble-dashed walls of number two suddenly sparkled; the garage door gleamed, and between them, Harry saw, quite distinctly, the hulking outline of something very big, with wide, gleaming eyes.”
Neither do Crucio and Imperio, since Harry casts both without ever finding out specific wand movements for either of them.