Photopea is a great FREE online alternative to Affinity. We use Affinity in school for editing photos, and we also have offered it to you to use at home. Unfortunately some students struggle to access it at home.
Please use the links below and watch the videos to learn the basics - you'll see it is very similar to Affinity.
Below is a selection of the amazing Photopea tutorials created by 'Mr E's Art Club' on YouTube. Be sure to click on his channel to find more which could be useful for the project you are completing.