Generally the main thing is show as many development stages towards your final piece as possible. You want to document everything from your very initial research - moodboards etc but also you might wanna include sections from any websites you read about your topic theming (with references) and any other research (even if it's just writing down parts of conversations you had with other people discussing your ideas).
This is something I wish i'd done more of.
Then initial sketches, drafting ideas, basic colour palettes, show some ideas for how the overall "shape" might be - sillhouette, forms, layout. That kind of thing. Maybe you know you want to include 3 key elements, try collaging some placeholder images to get a feel for their layout on the page. Include all of that in your development work.
Material testing they love to see. Even if you're set in your mind you want to do an oil painting (for example), do some experimenting with other paints, do a sample with oil pastels, do one with coloured ink etc. It shows you're more versatile and that oil paint was a calculated decision. Again, wish i'd done more of this - I did some but could've easily done more.
Artist references they will always ask for - find people doing work in a similar style and as well as doing "copies" which they will normally suggest (imitating one of their pieces) try to do something relevant to your theming in their style as well. So if one of your key themes was crabs but your artist reference mostly paints horses, you might paint a crab in the style of the artist's horse paintings.
When you do your final piece try and talk about how your development work has ultimately contributed to that final piece. What bits of inspiration did you take from those artist references? What did you learn from trying different mediums? Did trying different layouts and mediums result in a better outcome than what would've been your first idea?
That's the stuff that will get you an A*. Best of luck ❤️ I got a A* in Art & Design A level and an A in Graphic Design.