What is the meaning of "Nothing new will be said here, nor have I any skill in composition. Therefore I do not imagine that I can benefit others." To my knowledge, Bodhicaryāvatāra expressed the Nothing new will be said here, nor have I any skill in composition. Therefore I do not imagine that I can benefit others. I have done this to perfume my own mind..

In my limited understanding, it refers to the negative thoughts although I do not have complete comprehension of it. 

What does it refer to, what is the intention and what is to be practiced?
Shantideva’s work is among the most beautiful and profound pieces of writing ever composed on earth in any language. It is jaw dropping in its profound beauty and the author must have been an extraordinary Bodhisattva if not a fully accomplished enlightened being. Personally, I prefer to see it as the work of a fully and completely enlightened being. To me this verse was written out of compassion by this perfect being  to show others how to accomplish the same by overcoming the foes of conceit and self-importance.

The purpose of this verse is a thought training by the author intended to show others how to subdue conceit and lust for reputation. It is a verse which is full of humility and self-deprication designed to counteract conceited and prideful thoughts.

Shantideva's text is a master class in training the mind to be a Bodhisattva. All the verses are examples illustrating how a Bodhisattva practices and thinks.