Hello! I took gcse astronomy a couple of years ago, so can let you know what I remember but since my teachers submitted the actual work for us, I can’t comment on the exact process of submitting and marking etc.
It is compulsory to complete at least two of the observation projects on the website you provided (note it’s helpful to cross reference the titles with those on the actual specification as I think one or two have changed over the years). One must be aided (with a telescope/online telescope) and one must be unaided (naked eye). You don’t have to provide physical evidence of you doing these, but submit a document with the sections outlined in the documents on the website (your plan, photos/observations, analysis and conclusion). Make sure to put lots of detail into these as I think the examiners can be strict sometimes. Also note the coursework doesn’t contribute to your final grade (bit weird ik) but you must pass it to take the exams/pass if I remember correctly.
I will also add I think it’s a good idea to do the shadow stick experiment at some point, as it’s commonly mentioned in exams, so it is good to get practice with the maths involved+graphs! Also my teachers recommended doing one or two extra for practice+ in case one of the ones you submit is rejected for whatever reason.
Good luck and I hope this gives you a slighter clear idea, let me know if you have any more specific qs and I’ll try my best to answer if I can!