I like to work with Kanaya or Karkat's sprites, because they don't have anything obscuring their faces, but also because they're good "basic" male and female models.
I always work on separate layers, this way it's easier to correct a mistake, should I make one.
Modifying the facial features makes each sprite look unique. This way, the end result looks a little bit less like a "slightly modified Kanaya". In this case, the character was a ghost, which justified the white eyes. I worked with a sketch reference picture, which also lacked a symbol on the front of the dress.
For the legs, I'll sometimes grab a different set from another character. It's faster than making my own, and looks more true to the source material. Here, I took Rose's skirt and legs, put them in place by setting the layer as transparent, then modified them with the pencil tool.
The last two steps is how I get them to look transparent as a PNG, by setting the background layer as a new layer, then deleting the white that surrounds the sprite.
And there you go! Homestuck fantroll sprite!
Trolls and homestuck and base sprites all belong to one Andrew Hussie. Sablier is not my character either.