mettre la charrue avant les bœufs

To do things in the wrong order

Getting the steps of something backwards, doing a later stage before an earlier one.

Il a signé le bail avant même d'avoir visité l'appartement, il met la charrue avant les bœufs.

He signed the lease before even viewing the flat, he's got things back to front.

Simple farming logic: the oxen pull the plough, so putting it in front of them gets you nowhere. Similar phrasing turns up as far back as the 13th-century Roman de Tristan, and the version with 'charrue' and 'bœufs' was settled by the 16th century.