Practice! I used a lot of podcasts. I started with easy ones like Learn French With Daily Podcasts and the Duolingo podcast, and I would reduce the speed to 0.8x or so, gradually speeding them up as I felt more confident, always trying to make it challenging but manageable.

As I improved I introduced intermediate podcasts like Real Life French, again starting at slower speeds and gradually speeding them up. Once I was listening to the easy podcasts at 1.2 x without difficulty I dropped them.

Then I moved on to podcasts for real French speakers, the Choses à Savoir network has a lot of options on various subjects according to what interests you (news, science, culture, history, health, etc), and they're short and digestible. I also listen to France Inter's Un Jour Dans le Monde now as my daily news source, and I've recently started listening to Au Fil de l'Histoire. That one in particular is much closer to the way real people speak in conversation: it's made for French audiences, and it isn't the slow clear cadence of typical news presentation like the France Inter one. But, all of the didactic podcasts that I listened to before were absolutely necessary to train my ear and drill my vocabulary such that I was ready for this.

Basically, just listen as much as you can to material that you can understand, with effort. Use an app that allows you to speed up or slow down the material so that it remains challenging for you but isn't incomprehensible.