Start with this link: Dafny Insertion Sort. Use the Dafny system on-line, or download and install if you prefer, and work on it until Dafny says something like "Dafny program verifier finished with 3 verified, 0 errors."
Add a //README comment at the top describing who you helped (or got help from), and what kind of help. Copy-paste your entire solution into a file sort.txt, and upload it at right.