I cut 5 pallet boards to 3 ft and used the other ends to mount the boards together to form a shelf. One of the connecting boards I use as a guide to cut corners of the shelf is to lean my head wherever I want back in my chair. Then 3 quick angles cuts on the corners to round both points and shape into a rounded edge with a sander (small children around), and it flows better. Nothing precise, just cut and shape.
Small steps, you can always cut more, cut too much, and it could throw everything off, then you’ll find yourself slowly widdling away trying to compensate until your shelf isn’t a shelf no more. I used other scraps from the first cut and screwed them to studs to flush my shelf with my window seal. I stained it, found a lamp, and got my record player away from my 4-year-old son. Window, Chair, Music! What else do you need?