In my view, your writing is pretty good
Last Friday, I bought a T-shirt from your shop and the poor quality of this shirt makes me very disappointed.
1. Say what happened
First of all, the length of two sleeves is different, which makes me impossible to wear. In addition to this, the colour of the shirt does not match the sample that I have decided in your shop.
I have already discussed these problems with one of staff - Mr. Robert in your store by phone. But he was very rude and told me he can do nothing about it, as it was purchase at a reduced price.
2. Explain the problems
Unless you replace the door T-shirt, I will be forced to take a legal action.
Your earliest response would be highly appreciated
3. Say what you would like them to do
In my oppinion, it will be good if you give an advice for them what should they do to overcome a complaint.