follow the quick and easy tutorial to create a dome in sketchup, which can be used in 3d-printing and more

Step 1: Make a Circle

(make sure the diameter of the circle is a easy, whole number such as 6 or 8 rather than 5.67, etc)

Step 2: Make a Line

make a line going straight up starting at the center of the circle (if you put your mouse over the general area of the center a green center dot should appear) the height of this line will determine the height of your dome. (if you want a perfect half circle as your dome, make the height of the line the same as the diameter as your circle

Step 3: Make an Arc

use the arc tool (4 buttons over from the top left) to create an arch from the top of your line to the edge of your circle. (make sure it is along the green axis, the circle thing will flash green to indicate this)

Step 4: Make a Line

make another line from the bottom of your vertical line to the end of your arc. (this will make the ac turn white and become opaque, if it doesn't do this it probably is not aligned properly.

Step 5: Select the Follow Me Tool

select the follow me tool (it is under tools, as shown in the photo).

Step 6: Use the Follow Me Tool

select the arc with the follow me tool and run it along the edge of the circle. this will require some fiddling, and don't worry if its not flawless, just make sure it is a complete dome with no gaps. (regard next step)

Step 7: Delete Lines

you may get some random lines on your shape as shown, just delete them to have a flawless dome

Step 8: Your Dome Is Finished!

you now have a dome. the bottom may me hollow, so draw some lines on the bottom to fill it in (this is not necessary, the bottom may not be visible and if so you can disregard this step