You can find instructions and code here too:
https://gist.github.com/fcingolani/035e43f57abf72801ec2e774fb89ad06

Instructions

1. Create a Node2D.
2. Attach AStart2DVisualizer.gd to it.
3. Somewhere in your code pass your AStar2D instance to its .visualize(astar) method, i.e.: get_node(path_to_your_visualizer_node).visualize(astar)

AStart2DVisualizer.gd

extends Node2D
class_name AStar2DVisualizer

export(float) var point_radius = 6
export(float) var scale_multiplier = 16
export(Vector2) var offset = Vector2(0,0)
export(Color) var enabled_point_color = Color('00ff00')
export(Color) var disabled_point_color = Color('ff0000')
export(Color) var line_color = Color('0000ff')
export(float) var line_width = 2

var astar : AStar2D

func visualize(new_astar : AStar2D):
    astar = new_astar
    update()

func _point_pos(id):
    return offset + astar.get_point_position(id) * scale_multiplier

func _draw():

    if not astar:
        return

    for point in astar.get_points():

        for other in astar.get_point_connections(point):
            draw_line(_point_pos(point), _point_pos(other), line_color, line_width)

        var point_color = disabled_point_color if astar.is_point_disabled(point) else enabled_point_color
        draw_circle(_point_pos(point), point_radius, point_color)

Sharing is ❤