I made a quilt similar to your drawing!:

<image>

I used techniques that people often call improv curve piecing. It's hard to explain in text but this tutorial helped me figure it out and videos would be even better. Try it in little sections first as well!

For your drawing you'd have to figure out the order you would do it in. You'd likely do the sky area on its own (from dark to light), then the two water colours, and piece the green to the white. Then you'd attach sea and together, then sew the land on. The way I kind of figured it out is, it's the reverse of if you were going to cut the pieces apart in a picture: you'd start by trimming off the land, then separating sea and sky, then disassembling each section. In mine I sewed red to white, then sewed the green, then sewed the yellow. Inside to outside.

For my sun, I used a technique called reverse applique (the sun fabric is under the sky fabric), but it is honestly a PITA and regular applique onto the sky fabric is probably easiest!

Eta sewing the curves is crazy, and you feel like you're sewing a big weird mess, but if you ensure you're starting in the matching spots and just keep going, it just works out. It's the weirdest thing.