A sphere is a stack of circles. Each horizontal layer is a ring, the rings grow toward the middle and shrink again toward the top, and the whole job becomes manageable the moment you stop thinking of it as a curved surface.
Start in the middle
Build the widest ring first, at the height where the sphere is fattest. It is the easiest layer to place accurately, and everything above and below hangs off it. Working outward from the equator in both directions also means any small error stays symmetrical instead of accumulating.
Use a layer blueprint
Eyeballing each ring is how spheres end up lumpy. The sphere generator gives you every layer separately, with the layer below shown in pale blue underneath so each new ring has something to line up against. Step upward with the arrow keys as you build.
It also gives you the total block count split into stacks, which matters more than you would think — a 31-block hollow sphere is a serious amount of material to haul up scaffolding.
Size and material
Below about 9 blocks a sphere looks like a rounded cube. From 15 it reads as round; 25 and up is smooth enough for domes, planetariums and giant fruit. Hollow shells are what you want for anything you will stand inside; solid is only for terraforming or for carving out later.
Blocks with subtle texture variation, like stone bricks or deepslate, hide the stepping better than flat single-colour blocks such as concrete.
Scaffolding saves hours
Build a central pillar first and work outward from it at each layer, then remove the pillar at the end. Trying to place the upper layers from outside means a lot of falling.
For half spheres, the dome generator does the same job without the wasted lower layers.