The fastest way to create cubemaps is to import them from specially laid out Textures. Select the Texture in the Project window, to see the Import Settings in the Inspector window. In the Import Settings, set the Texture Type to Default, or Single Channel, and the Texture Shape to Cube. Unity then automatically sets the Texture up as a Cubemap.

Several commonly-used cubemap layouts are supported (and in most cases, Unity detects them automatically).

Vertical and horizontal cross layouts, as well as column and row of cubemap faces are supported:

Another common layout is (Latitude-Longitude, sometimes called cylindrical). Panorama images are often in this layout:

(spherical environment map) images can also be found:

By default Unity looks at the aspect ratio of the imported texture to determine the most appopriate layout from the above. When imported, a cubemap is produced which can be used for skyboxes and reflections:

Selecting option is useful for cubemap textures that will be used by Reflection Probes. It processes cubemap mipmap levels in a special way (specular convolution) that can be used to simulate reflections from surfaces of different smoothness:

Another useful technique is to generate the cubemap from the contents of a Unity scene using a script. The Camera.RenderToCubemap function can record the six face images from any desired position in the scene; the code example on the function's script reference page adds a menu command to make this task easy.

Unity also supports creating cubemaps out of six separate textures. Select Assets > Create > Rendering > Legacy Cubemap from the menu, and drag six textures into empty slots in the inspector.

Note that it is preferred to create cubemaps using the Cubemap texture import type (see above) - this way cubemap texture data can be compressed; edge fixups and glossy reflection convolution be performed; and HDR cubemaps are supported.