BUT HOW DOES SHE PUT IT ON?!

She grows into it.

The power to shrink/grow must be really handy.

She disperses and reappears with it on, or she makes it disperse and reappear on her head.

[Super Stretchy Suika Head Warmer]

Have you ever thinking about how she sleeps? I usually lie on my right side. She can only sleep on her back or needs a hole through her pillow and bed.

It stretches.
As for sleeping, she can lay her head on bed's edge, with horns being off the edge.