A bad beginning makes a bad ending

“A bad beginning makes a bad ending” is a strong equivalent for “À mal enfourner, on fait les pains cornus.”

The French proverb means that if you handle something badly from the start, the result will probably go wrong. The Académie française gives exactly this explanation: the failure of an undertaking often comes from having gone about it badly at the beginning.
The image comes from baking: if the bread is badly put into the oven, it comes out misshapen, or cornu. So the proverb does not really speak about bread, but about the importance of starting properly.

“A bad beginning makes a bad ending” works well because it keeps the proverb-like form and preserves both ideas: the bad start and the bad result.

Other possible translations:

A bad start often leads to a bad outcome — very faithful, but less proverbial.

If you start off wrong, things are likely to go wrong — natural and clear, but more explanatory.

To get off on the wrong foot — idiomatic, but incomplete, because it only means “to start badly,” without necessarily expressing the bad final result.

Well begun is half done — close idea, but positive rather than negative.

So the best compact translation is:
À mal enfourner, on fait les pains cornus.
→ A bad beginning makes a bad ending.