Great product, just little to no instructions. Therefore for those struggling a little - I used a sharp butchers knife to prise the cap off the vertical bar (you need something sharp to get between the cap and the bar), then used a mallet to get the boom arm stopper wedged in properly. Then tighten the nut on the stopper with a spanner, place the cap over the thread/nut, slide the boom arm out of the holder and screw the holder to the stopper as tight as it’ll go, then it’ll rotate smoothly with a bit of torque force from the reattached boom arm. Job done.