If you didn't find a returns label in your package, then you can print one off from the website. Wrap your parcel (you can reuse the packaging it was sent in using extra sellotape), put on the returns label and take it to the post office. Assuming you are in the UK, postage is free. In around 21 days you should get a refund.
https://www.prettylittlething.com/returns-policy