Making a randomizer script for a basic "Runner" game...

Which way is best really depends on the game, but a simple, general way would be to have a set of level "chunks". Whenever you need a new chunk (like when the player is getting close enough to the end of the generated level so far), just choose one from the set randomly and append it to the end of the level. Each chunk would be different, so the level would be different each time it's played. There's all kinds of complexity you can add from there, depending on the game.