Arya didin't feel like she need to practice the wand movement since it was a simple flick. So when she finished pronouncing the incantation, she separated one of the worms from the others and concentrating on a mental image of a nail.

A nice shiny nail.

"Tacque." She flicked her wand, and watched as half the worm slowly turned into a nail. The other half vehemently wiggle around. Creepy. A frown crossed her expression as she quickly flicked her wand again. "Reverto."

This time she took longer to let the image of the nail sink in. More to get rid of the image of the worm!nail, and to let the worm she was working on get a little break from being transformed. After a moment she was sure she would hold on to the image, she flicked her wand again. "Tacque."

The worm almost immediately transformed into a shiny silver nail. Picking it up with her free hand, a satisfied grin on her face, she inspected the nail to make sure it had transfigured completely. Didn't want a squishy nail.

It was completely transfigured.

Placing the nail back on the desk, she reverted it and continued to practice transfiguring the worms into nails.