When you were introduced to the instrument, did someone show you how to safely assemble it?

Flutes have three sections:  head joint, body, foot joint.  The head and body have tenons … the thinner metal at the bottom of the joints … that fit into the thicker sections at the top of the joints.  The part of the body into which the head tenon fits is called the barrel.  The barrel and the foot joint section into which the body tenon fits are both much thicker than the rest of the instrument.  So the thinnest two sections of the instrument fit into the thickest two sections of the instrument.

When you are ready to fit the sections together, gently remove head joint and body from your case.  Hold the body by the barrel .. NEVER with your hand over the rods .. hold the head joint with your hand covering the embouchure plate (which has the hole cut in it).  CAREFULLY align the head joint with the body vertically, as shown in the flute on the left.  Gently insert the tenon into the barrel, then twist it to move it further into the barrel.  DO NOT tilt the head joint at an angle or align it sideways and “jam” it into the body.  Doing so with bend the tenon, causing several issues.  After the head joint is in position, hold the instrument by the barrel (now your hand will be partially on the barrel and partially on the head joint), insert a finger into one end of the foot joint, and lift it from the case.  DO NOT pull on the rods, or put pressure on them as you align the foot joint carefully with the body tenon.  Gently insert the tenon into the foot joint, twisting to mate it fully into the foot.  Turn the foot joint so that the rods are approximately aligned with the center of the body keys.