1. Open Keyboard Settings and click on Layout Settings. Alternately you can also directly open Keyboard Layout settings from System Settings.

2. Under Layouts tab, click on Options button.

3. Expand Compose Key Position.

4. Assign a compose key (say Right Alt) by clicking on checkbox.

5. Close the settings window.

Special characters can be inserted by holding down compose key and pressing the combination buttons.

For Example:

©     Right Alt + o, c (Press and hold right Alt, then press o and then c while holding down right Alt)

™    Right Alt + t, m (Press and hold right Alt, then press t and then m while holding down right Alt key)

®     Right Alt + o, r

Note:  1. Do not release Compose button until special character appears.

2. Full list of combination key strokes can be found in /usr/share/X11/locale/en_US.UTF-8/Compose