How do Changeling transformations work?

[ggg-2 3,553]

Thorax's "Crystal Hoof" disguise got me thinking about their transformation. How does it work? Do they have to see a pony and use it as a reference or can they just make somepony up?

[cmarston1 5,959]

I think they can just make someone up for a disguise. But given how changelings were before season 6, it made sense for them to use others as references in order to gain the most love.

[digit112 89]

I guess it's like any other spell, they can control how they look just like how we can control how we move. It's not difficult to understand, just to explain. They can probably make any pony they want, but will obviously have to see some pony to copy them.