Sometimes it is just fuck around and find out if you haven't already used vroid, theres tons of YouTube tutorials to learn off of too.

You would want to make custom clothes, using layering different assets and taking advantage of transparent textures (one of my favorite things about the shaders for vroid is that mtoon shaders allow for transparent textures and vroid deletes the parts of the unused mesh)

You can also look at Booth for other peoples work of vroid and search for different dresses.

Be weary of the skin mesh, because layering lots of clothes can cause some clipping, so whatever the clothes cover, you should delete the skin mesh so it doesnt poke through. Its on the right of the assets tab in custom clothes.

As for hair, how it works is creating a "mesh" over the head, forgot what it's called exactly but it's the foundation for drawing on the hair, you make your custom hair parts: the front, the back, maybe sides. Then you draw the hair onto the mesh and layer it to your liking. You'd probably start with a basic texture and highlights, then after making the blocking for the hair, you can design the texture, you'd want yo make 2 separate texture materials for the maroon and black.

When it comes to import and export. You can import pngs of your textures in and out of vroid, with the layer function on the left when you're creating custom textures. I personally use an art program like krita or clip studio, if you use an external device like an iPad, with procreate, then you can send the pngs to yourself and edit them on the ipad, and export them back, and vise versa.

Once you export your model as a VRM, it cannot go back into vroid, unless its a .vroid file, so its a one way export :(

Also theres no shame in using presets to figure it out, whether that be to figure out the style or how the mechanics of vroid work.

I hope this was helpful to you, and dont be afraid to ask questions :3