Take the training wheels off and also remove the pedals. Lower the seat way down so she can very easily put both feet flat on the ground while seated (borrow a smaller bike if necessary). Then follow these steps:

In an empty, level parking lot (corporate/school lot on weekends, church lot during the week, etc.) have her practice sitting on the seat and just walking the bike along by pushing off with her feet. This is much easier without pedals that could otherwise hit her legs.

Gradually have her practice taking longer steps between putting a foot on the ground. Remind her that if she feels the bike leaning to one side she can steer in that direction to stay up - but that she can also put a foot down anytime she's the least afraid of falling. Don't be concerned about going in any particular direction at first - just on taking longer and longer coasts between steps.

Once she seems comfortable with pushing off and coasting then have her practice going in a straight line to some specific point. Most parking lots have a bit of a slope for drainage and this is often enough to coast at about walking speed - use that to let her coast for longer distances and practice both straight line riding and making a few turns.

Only when she's really comfortable using the bike this way as a 'hobby-horse' should you put the pedals back on. But leave the seat down low so she still has the security of being able to put a foot down if needed. Efficient pedaling can come later. Have her practice starting by pushing off with one foot and then continuing by pedaling.

Finally, start raising the seat up to a more normal level by small steps as she gets more confidence.

In my experience this process takes only about an hour once the child is mature enough to learn balancing and rarely involves even a single fall. It also works well for teaching adults who never learned to ride as children.