Hello. First, in Blender import the stl file you get here. Select the item. Then go to Edit mode. Then go to faces. Press "z" to go into xray mode. Select all the faces of the part you want to print. Then press "p" to separate your selection, making it its own object. Then in object mode, select your new object, and export it as its own stl file. Make sure you have the option "Selection Only" checked off. Boom! You got the stl file you need to print.
If you haven't already, take some tutorials on the basics of how Blender works. Once you get this basic stuff down of how to select the different modes, how to select objects, and how to move the camera you'll be fine. Good luck!