AS Level Nucleophilic Substitution Question?

I dont understand the mechanism for the ammonia acting as the nucleophile. I have been through the mechanisms for the hydroxide ion and the cyanide ion and understand both these because they are simple. But the ammonia mechanism has so many steps and other arrows are introduced which I dont understand why they are there.Does anyone have a simpler version which explains why certain arrows are in the place they are and why therwe are negative or positive signs in places and what these mean. Basically, any help is appreciated

Thanks.