Look up pro forma in a dictionary. This can be an actual book or an online dictionary. When you find it, read every possible definition and connotation of the word. This will make it easier to determine its appropriate use in a sentence.
Research pro forma in a thesaurus to deepen your understanding of the meaning. By seeing similar words, you will understand the concept of pro forma better and you will be able use it more appropriately in a sentence.
Write down the various sentences that come to mind. Refer back to the dictionary to determine whether you have captured the true meaning of the word.