That's the formula for the envoi (the final three lines in Sestina). You have to label each word as a, b, c, d, e, f. So the end words are in that order, then the order fluctuates as you go- I had to go back to my book of forms a few times to make sure I was correctly following the structure! In the final 3 lines you have to use two keywords, b,e for line 37, d,c for 38, and f,a for the last. Thanks for the insight and feedback!