Until Kodi gets a native frontend for selecting games the easiest way is to use the Internet Archive Game Launcher addon by zachmorris Internet Archive Game Launcher
A lot of more info is available in the retroplayer section of the Kodi forum Kodi Community Forum - RetroPlayer
Basically you just have to install a mame addon from the LibreELEC repo and setup your controller. Then use the game launcher or enter the konami code, setup advancedsettings so that zip etc files aren't associated with audio/video files and select ROMs via the file list in the Games section of kodi and open them via the context menu / play with...
so long,
Hias
Until Kodi gets a native frontend for selecting games the easiest way is to use the Internet Archive Game Launcher addon by zachmorris Internet Archive Game Launcher
I though that Leia will get also proper game front-end support (incl. Game selector), but seems I was wrong (mean my expectations).
Has that (native game-selecting front-end):
1) Never been on Leia roadmap (= not planned)?
2) Not in scope for the 1st "release"
3) Initially planned for Kodi "M"? (yeah I know, perhaps wrong place to ask) or
4) Front-end game-selector needs to done via a separate add-on in Kodi Leia
Appreciated 
//t
AFAIK a Kodi game frontend (plus all the kodi backend database stuff needed to manage this) had been planned for Kodi 18 / Leia but retroplayer / games turned out to be more work than expected, so this had to be moved to Kodi 19.
so long,
Hias
It is shifted to k19/20 because the current db codebase is so messed up that an rewrite of the db is needed to make an RP integration possible 
Ouch... what a disappointment
But yet, I think it's better to do things properly than try to rush.
Any place to follow that progress (roadmap / gitbub etc.)?
//timo
No roadmap available, its more a personal agenda of the devs.
Understood & shall keep on waiting.
Most likely will "jump along" with the IAGL or Retroplayer add-on, no patience to wait until Kodi 19/20 
I think there are a bunch of "How to play games in LE / Kodi with xyz" already, but I think it would be great to have somekind of summary with the upcoming LE release written in the Wiki perhaps?
Just a thought.
//t
I just gave the Leia Game AddOn some tries, but failed each time.
Scummvm didn‘t even start.
DOS BOX started and left me with a black screen.
Vice 64 (Using IAGL) also started but hung with the loading message.
