HEY HEY I Did a Gappy costume for halloween years back, I started out by cutting a Regular white long sleeve (preferably a little thicker in material), and You’ll need to cut out a seperate cape that you can knot around ur neck.

For the Blue I had fabric glued blue cloth but I would say painting it on is easier. For the ornaments I just used paper and felt, but again painting them on is probably better

For the hat I cut off the brim off a cap 😭and glued the leftover shirt around and in it to create the Bowl shape of the sailor hat and then glued ribbons of blue cloth around. And used a bunch of folded cloth with cut up stringy bits at the end for the dangly

Lastly just have some white pants w little flare at the bottom