Summarize:

Emoji is a non-standard set of characters, and is not supported by all systems. Both you and your recipient must have emoji support in order for them to be seen by both sides.   iOS - All iOS devices running iOS 5 or later have emoji support built-in. The emoji keyboard may need to be enabled. Click here for instructions.  Android - Not all Android devices support emoji, though certain apps such as Hangouts and WhatsApp do regardless of device. To add emoji support to your Android device for all apps, click here.  OS X - OS X has built-in emoji support since OS X 10.7.   Windows 7 and earlier - Emoji support is based on the web browser, so make sure all of your browsers are updated to the latest version.  Windows 8 - Windows 8 includes a built-in emoji keyboard. To enable the keyboard, open Desktop mode, right-click on the taskbar, and select "Toolbars" → "Touch Keyboard". You'll see the keyboard icon appear next to the System Tray. Emoji symbols are added by selecting the specific symbol you want instead of typing a set of characters. The process for selecting the symbol differs depending on the system you're using.    iOS  - After enabling the emoji keyboard, tap the Smiley-face button when the keyboard is up to open the Emoji keyboard. If you have more than one language installed, the button will be a Globe instead of a Smiley-face. Scroll through the options and tap the one you want to add.  Android - The exact method for opening the emoji menu depends on the version of Android you are running and the keyboard you are using. Usually you can tap the Smiley-face button, though you may have to press and hold a button to make it appear instead. Scroll through the options and tap the one you want to add.  OS X - In 10.9 and 10.10, you can press ⌘ Cmd+Ctrl+Space to open the emoji selection window. In 10.7 and 10.8, click the Edit menu in the program you are using and select "Special Characters". Click the Gear icon and select "Customize list". Check the emoji box to allow emoji characters to be selectable.  Windows 7 and earlier - If your browser is up to date, you can copy and paste emoji from various emoji databases like Wikipedia. There is no way to type emoji characters.  Windows 8 - Click the Keyboard button that you enabled in the previous step. Click the Smiley-face button at the bottom of the keyboard to open the emoji menu. Click the emoji that you want to add.
Determine if your system or program supports emoji. Add emoji symbols to your text.