Trace the shape of the ring blank onto the index card.

Paint the platform of the ring blank with black nail polish and set aside to dry.

Start coloring in the trace:
I started with the darkest color first (purple) and added on layers of the others.

Shading lightly is best so you can add instead of trying to erase and making a big globby galaxy mess.

(I looked on the how-to for the Galaxy Leggings by TheHappiiZombie for reference)

For the comets:
Grab your handy toothpick and lightly dip the flat circular end into some white paint.

Place it on your galaxy and drag.

For the stars:
Dip the pointy end of the toothpick into white paint and dot your galaxy. Maybe add some yellow stars as well.

(How-to for Space Shoes by raincloudriot for reference)

After some drying, cut out your galaxy and glue it on the ring blank and then the glass bubble.

Make SURE the paper is fully dried or else the glue will attack it and the glass bubble will decide it wants to shuffle around and you'll end up with a messy glob.

Drying time may test your patience.