There are two ways to reduce a relative clause to a phrase:
1. By omitting the pronoun and changing the verb to an "-ing" form
2. If the clause contains "be + adjective", move the adjective in front of the noun
The document provides examples of reducing clauses like "who is wearing" to phrases like "wearing" and "that are new" to "new". Care must be taken as only clauses with pronouns like "who", "which", or "that" can be reduced.