Go to your Adobe Illustrator window. Create a new workspace to work on. Now go to the top of your window and click Window > Brush Libraries > Other Library. When the box opens, you will need to locate the brush that you just downloaded. Click the brush file that you downloaded and click the open button.
A new box will open in your Illustrator program with the brushes that you just downloaded.
Click the line that you created to select it. Now click one of the brushes in the box that you just opened. The line has now become the brush that you applied to it.