Give your students a list of formal words and phrases in Spanish. Examples include "le agradezco (thank you)," "la entrevista (the interview)" and "cordialmente (cordially)." Ask each student to compose her own formal letter utilizing a specific set of the words and phrases. Alternatively, give the students an already-composed letter which is missing some of the formal vocabulary. Have the students fill in the blanks with the suitable words and phrases.
Come up with some different functions and purposes of formal Spanish letters, such as inquiring about a job opening, writing a cover letter, or replying to a request for information regarding a job. Write each function on a scrap of paper and put the scraps into a bag. Ask each student to pick a scrap from the bag and write a letter fulfilling the function. After the students have written their letters, have them read the letters out loud to the class.
Most companies probably don't have the time to correspond with students solely for the purpose of learning Spanish. In light of this, ask your students to exchange email addresses or postal addresses with each other. Have them exchange formal correspondences. They can write to each other regarding the topics covered in class, or regarding topics of their own choosing.
Give your students a sample formal letter that contains various mistakes in spelling, grammar and structure. Ask the students to correct the mistakes. After the students have corrected the letters individually, ask them to compare their corrections with each other. As a whole class, check that the letter is error-free.