When you submit your claim, make sure you keep a copy of your claim form. You will need it later.
If you’re going to post the form, photocopy it before you do so.
If you submit it online, make sure you download a copy (either print it out or save it somewhere safe) at the end of the process. One potential pitfall is that, if you attach a document to the form online, the copy you download won’t include the attachment. You need to save / print it separately.