Mirror Modifier will duplicate half of an object across its axis which means if we change one side of an object then Blender will fill in the other half for us. Alright, without wasting time lets get the tutorial started. Follow below steps to create a sphere and modify it together with Mirror Modifier.
1 Open up your Blender software then delete the default cube.
2 Add a UV sphere from Mesh.
3 Switch to Edit Mode from Object Mode.
4 Switch to Face select and select Wireframe from Viewpost Shading then select the left half of the sphere by pressing the B key and drag the cursor to cover the entire left half of the sphere.
5 Press x to delete the selected faces. Switch to Front Ortho View and press A to select the other half of the sphere.
6 Next select the Modifier tab from the properties panel.
7 Select Mirror Modifier.
8 Under Mirror Modifier select X Axis, then select Merge, Clipping and Vertex Groups under Options. Enable the Clipping option will ensure our vertices do not cross the centerline of an object.
Also select the triangle button as shown below.
9 That is it, now you can modify the right half of the sphere and the left half will automatically matches the right one.
This tutorial will not repeat those features that I have already mentioned in my previous tutorials, so for example if you want to know how to switch from Object Mode to Edit Mode then please read my previous tutorial.