A dome is the top half of a sphere, which makes it considerably easier than a full one: you never have to place blocks underneath yourself.
Work bottom up
The widest ring sits on your walls. Each layer above steps inward slightly, and the last one is a small cap. Build every layer completely before moving up, because reaching back down through a finished shell is awkward.
The dome generator gives you each layer as a blueprint with the previous layer ghosted underneath, which is the part that stops domes drifting off centre — the usual failure when people build them freehand.
Choosing the proportions
A perfect dome is exactly half a sphere: height equal to half the width. That looks right on observatories and cathedrals. For a lower, wider roof, switch off the perfect setting and reduce the height — a 31 wide by 10 tall dome reads as a shallow roof rather than a bubble.
Where domes go wrong
Two mistakes account for most bad domes. The first is starting from the top, which guarantees the base ring will not match your walls. The second is not counting: on a large dome it is easy to step in one block too early on one side, and the asymmetry is glaringly obvious once the shell is closed.
Finishing touches
Swap the outermost block of each step for a stair facing outward and the stepping softens dramatically. Glass panes in a ring near the base turn a plain dome into a greenhouse or an observatory. Leave the cap block until last and place it from inside on scaffolding.
For flat rings and floors underneath, use the circle generator.