[BeepoGirl]

How does it act as a base and how can I draw the reaction in terms of curly arrows?

[StarvinMarvin]

BuLi acts as a Lewis base simply being a donor of electron pair. So it is equivalent to something like Bu- but you have to remember that the nature of the bond between C and Li is more covalent than ionic. In terms of curly arrows, the bond between butyl group and lithium BuLi is the one attacking an electrophile.

If you like I can draw it for you.

[BeepoGirl]

Ah, thanks that helps a lot. But no I don't need a diagram.