How would you use sojourner truth as a common noun?

You can't. Sojourner Truth was a person by that name so it has to be a proper noun.

The noun 'sojourner' is a common noun as a general Word for someone who stays in a place for a short time and moves on.

The noun 'truth' is a common noun as a general Word for what is believed to be reality, fact, or actuality.

Example sentence for the common nouns:

"I met a sojourner at Denton who tried always to tell the truth."