Determine whether the two clauses you want to join are dependent or independent. An independent clause is one that can stand on its own as a sentence, while a dependent clause cannot. Only two independent clauses can be joined together with a semicolon. Remember, too, that even though they are independent clauses, they also need to be related.
Write out the first clause as you would a regular sentence. For example, "Anne was very punctual." However, instead of placing a period at the end of the clause, use a semicolon: "Anne was very punctual;"
Add the second clause after the semicolon. Begin this clause with a lowercase letter and end it with a period. You now have two independent clauses joined together with a semicolon: "Anne was very punctual; she was never late for an appointment."