It’s really easy to attach the arms to the shield. they are different for the left and right side. it’s obvious which is which.
Where they attach to the shield there are two holes that match the little pips on the arms. Put the top of the arm into the slot at an angle so the top pip goes into it’s hole.
Now use your thumb to push up the bottom pip and then bring the arm down so it pushes the pip the slot until it clicks into the bottom hole.