Robin L. Simmons
Grammar Bytes: The Relative Clause
The section of this site dedicated to the relative clause provides the user with skills to be able to recognize relative clauses through a list of relative pronouns and relative adjectives along with sample sentences and examples.
Bartleby
Bartleby.com: The King's English: Grammar and Punctuation
Part of this chapter from the 1908 book by H.W. Fowler provides instructions for punctuating adjectival and adverbial clauses. However, there is a lot of other material on this page to scroll through.
Sophia Learning
Sophia: Adjective Clauses: Lesson 5
This lesson introduces adjective clauses and their placement in a sentence. It is 5 of 5 in the series titled "Adjective Clauses."
Other
University of North Carolina at Pembroke: Adjectivals
This page from the University of North Carolina at Pembroke provides instructions and examples for using different types of phrases (prepositional, appositive, and participial) as adjectives in sentences.