If you wear the crimson and gold common to staff appointments (e.g. Adjt, ADC), then it's worn on the left shoulder, buttoned beneath the shoulder board and with the metal dangly bits swinging around in front to make a mess of your Sam Browne. In SD / Mess Dress, the loose end clips to a hook beneath your left lapel (see your tailor), and in blues / Tunic, you clip it into the lower collar hook. A photo from the RMAS website of Ac Adjt demonstrating, and of FM Bramall showing the alternative thick gold rope of Royal appointments, worn on the right shoulder. If you are wearing both, you probably shouldn't be.
Unless of course you mean shoulder boards as in RLC Mess Dress and the like.
Apparently the cord should wind under not over at the front extremes, don't ask me why I am not on the dress Commitee! As long as you don't forget to remove them prior to dry cleaning you will be okay. (Unless you like long gold bits of cord that have no further use!)