From Longman Dictionary of Contemporary Englishput the cart before the horseput the cart before the horseto do two things in the wrong order → cart
Examples from the Corpus
put the cart before the horse• This is putting the cart before the horse.• It seems to me that Mr Topolski is putting the cart before the horse.• This may sound like putting the cart before the horse and being unnecessarily pessimistic.• It is Labour's insistence on putting the cart before the horse which fills me with gloom. 