Hi Laura
We have a guide that explains the process. It is the 36-page Guide to PIP appeals.
benefitsandwork.co.uk/guides-for-claimants/pip (scroll down the page). You can either fill in a form and send in your appeal or fill in an appeal online. The information starts on page 9. If you do it online - you won't miss the deadline. Don't worry if you don't have all the information you want immediately, you can always add information after you have lodged the appeal.
BIS