Writing An Effective Letter Of Complaint

[Willy Nilly]

I have a sports watch which I think is a load of guff and it annoys me every time I use it. I said as much on one of the brands posts relating to this item on a leading social media site and they want me to PM them with my issues.

So how do I get across my issues with this piece with out coming across as a moaning idiot so they listen, take note and do something about it?