In a soliloquy a person reveals their true thoughts to the audience. Think about what you would do to make it apparent to the audience that you feel a certain way.
So for example if I was sad maybe I'd use stark and dull imagery. If I was elated I might use more positive dialogue.
You have to engage with the audience. This can easily be done by asking rhetorical questions.
Just state your opinion clearly so that the audience will be able to know what you are going through.
Also, don't be afraid to use colloquial or standard English. Remember you are trying to make your soliloquy engaging.