The CORE digital attachment comes with two tubing connector sizes, a pediatric and a standard. The end of the tubing connectors is covered with blue tape. Please do not remove the blue tape.
You will need to thread in the connector to your CORE digital attachment in order to assemble your stethoscope.
First, thread in the connector with the blue taped side into the top of the CORE digital attachment. You can locate the top of the CORE digital attachment by finding the side closer to the power button.
Then, push in your chest-piece onto the free side of the tubing connector without the barb. See figure below.