Practice replacing nouns in simple sentences with noun clauses. As an example, the simple sentence "He should be hired." can be replaced with "Whoever is most qualified should be hired." where "whoever is most qualified" is the noun clause.
Practice using noun clause connectors to insert noun clauses into simple sentences. Use connectors such as "that," "what," "wherever" and "whenever" to connect noun clauses. As an example the sentence "He can come home today" can be changed to "He can come home whenever he wants to."
Practice changing questions to sentences with noun clauses. As an example, "Who is that man?" can be changed to "I wonder who that man is." In this instance "who that man is" is the noun clause.
Practice using a noun clause as the subject of a sentence. As an example, in the sentence "I must decide which dress fits me the best," "which dress fits me the best" is a noun clause and the direct object of the sentence.