Considering that Jaden had yet to learn how to play the instruments, he had to depend on some of the workers in the store that were capable of playing the instruments.
Jaden was not trying to create something professional, but instead, it was just a trial. So, even if the quality of the composition was not going to be that good, still, it would give him an idea of the current capability that he possessed.
In any case, theoretical knowledge did not imply that a person would be capable of doing it practically. So, it was only after he had finished composing a song would he really know the current level of the skill that he possessed.
The composition took quite a while. Considering that Jaden already possessed an idea, it was not that problematic for him to be able to use the knowledge that he possessed to convert the idea into the composition.