What preparations can I do before I paint the ceiling of my bathroom?
Any home improvement centre sells two essential things for this: painter's tape and plastic sheeting. Buy a thin plastic for the walls, and thicker one for the floor (or, better yet, use drop cloth for the floor). Buy wide painter's tape, it's much easier to use it to attach plastic sheets to walls, shower doors, etc.

Tape and cover everything. There will definitely be splatter, so cover every square inch of every surface. It will take only a half an hour or so, for a bathroom of this size, especially if you have someone to help you to spread and hang the plastic.
Just hang plastic sheets from top of the wall to the floor, completely covering any wall-attached fixtures, etc. Use painter's tape first to tack the sheet to the wall, and then use a long strip to completely cover the area where the wall meets the ceiling. Make sure sheets overlap, so there is complete coverage.

Cover the glass shower walls completely, from both sides. Again, just hang a sheet of plastic on them, attaching it with painter's tape.

Cover the floor completely. Use drop cloth, or thick plastic sheeting. Drop cloth is preferrable as it's not slippery, and will absorb any paint splatter or minor spills.

Pull out potlights' trim. They are usually just spring-loaded, and will go out if you pull a bit. No need to remove the pot lights, and don't cover them completely so they won't overheat, but pull the trim out so you can paint around their holes freely.

Use a brush to paint corners and along the wall border.

Use a pole and a roller to paint the ceiling (and wear a hat, otherwise your hair will be full of paint splatter. Don't ask how I know this).