回複：請教法居士及前輩，由婚姻申請綠卡，雇主證明信怎樣寫？

it is very easy and simple. the employer's letter should include the employee's name, salary, position in the company, spouse's name, starting date in the company... just some common information to prove that this person is real.
actually the immigration will give you a instruction and tell you what should be written in the letter.