Tested on Ubuntu 15.04 x64 but should work for any OS with OnBoard onscreen keyboard.
To display emoji you need to install font, which contain emoji symbols. Run command:
sudo apt-get install ttf-ancient-fonts
will install «symbola» font, which contain most of Unicode 8.0 emoji.
Also you can install other emoji fonts like Noto Emoji or EmojiSymbols.
Now copy all layout folder content to ~/.local/share/onboard/layouts folder.
Then Go to System Settings -> Accessibility and in Input tab enable onscreen keyboard.
In layout select «emoji» in user's layouts.
I prefer open emoji keyboard by hotkey, so i disable auto show keyboard on input and autoclose on hardware keyboard press. It can be done on general OnBoard settings. And in System Settings -> Keyboard you can set keyboard shortcut which run onboard command. Play around with OnBoard settings to achieve most useful configuration.
Layout .svg template contain grid for 12x9 symbol buttons and for 16 panel switchers which allow display up to 12x9x16=1728 emoji. .json file contain obvious configuration for panels. .onboard.template contain common .onboard structure to generate layout. .py script generate new .onboard file in layout folder based on this config and onboard template.
NOTE: some emoji contain more than one symbol (including invisible characters) so be careful to operate with it.
How to modify .svg template you can read in your /usr/share/onboard/docs/layouts.html