For me leadership is a mindset. I’ve seen too many team leaders that are simply super team members in my book. They may have knowledge, work hard, etc but can’t inspire others or strategize to remove obstacles for the team. Leadership is a mentality. They’re the people who are willing to ensure the person to the left and right of them are taken care of. Making those around them better and driving to make sure their lives are bit easier.
I say that to say this, demonstrating leadership first and foremost should be the start. They should be a leader amongst their peers. How to help with that? Put them in the position of leadership.
I remember when I was running a tough remodel I had team members basically running the set process for the day. I rotated it and seen how they performed and communicated. Many were able to get the buy in from the others and could inspire them to perform better. 5 promotions came from that team.
As others said above, you have to ensure their accomplishments are highlighted. Additionally, workbench has great tools to assist.