Retrosmc

First log into your osmc installation via ssh:

$ ssh osmc@your.device.ip.here

password hint: osmc

First, stop the media center. This will stop the media center and free up even more resources.

$ sudo systemctl stop mediacenter

Next download the installer:

$ wget https://github.com/mcobit/retrosmc/blob/master/install-retrosmc.sh

After downloading, make it executable and run the script:

$ chmod +x install-retrosmc.sh
$ ./install-retrosmc.sh

Select the first option. The installer should check and download some packages, You will eventually arrive at a screen to install Retropie. Go ahead and select the install option. The installer should mention that the user osmc isn't part of the input group; agree to add it.