How Do You Write Clearly?

To write clearly, you have to shape your words for the audience that will read them.
Are you writing a technical manual ? A children's book ? A romance novella ?
Try not to use words that the general public would  not understand.

My inamorata was morose or My fiance was sad ?

Or was she gloomy ? There are many ways to say it.

Write what you want to say and then read it to yourself.
Then read it again out loud. Is the meaning clear to you ?
Put it down and go do something else.
Come back to it later and read it again. Is it OK or do you want to revise it ?
If possible, let someone else read it without telling them anything about it.
Do they understand what you wrote ?

Revise your writing and make it the best you possibly can.