Having noted down everything the Professor told them, Sinead set her things aside so she didn't knock everything over (she really did not need ink spilled all over her notes) and readied herself for practice. She didn't want to mess this up and embarrass herself in front of everybody- so instead of trying to cast the spell at first, she opted for rehearsing the wand movement and pronunciation separately, watching the other students who had succeeded around her out of the corner of her eye. Any little difference in the angle you held your wand or the emphasis of the word could change the spell massively, she found- and it was easiest to let other people make those mistakes for you first.

When she thought she had the basics of it down, she aimed carefully towards the chest and moved her wand in the required arc with a crisp,
"Accio!"
The entire trunk shifted about half a centimetre in her direction. Right. Intention, not just aim. Good thing her spell didn't have enough power behind it to actually move the trunk; that would be horrifying. Steeling herself mentally, she focused on one of the snowballs to the exclusion of everything else and prepared herself to cast again. Second time lucky, wasn't it?