How to do level design?

That is a vague question. How about if you try to do it, and tell us when you reach a specific problem.

Do you know where to begin?
Is it transferring theory to practice?
Which specific issue are you tackling on at the moment?

You probably want to start with some basic requirements:

What do you need or want the player to experience in this level? Are there certain items to collect? Are they trying to kill a certain target? Running for their lives?

Does the storyline (if applicable) dictate certain elements? Is the player already at some point thanks to a previous level? Do you need them to reach a certain point for the start of the next level?

Will there be different outcomes possible, or only one possible ending to the level? Should the player be able to reach the ending(s) multiple ways, or only follow a single path?

Should the level be difficult, or was the last level difficult and the player needs a bit of a break? Should the player be scared? Stressed? Amused?