Guys, I don't understand how I can make a white (base) from a blue weapon (claws)?
Where i can find base?

You cant.
Base for a runeword can only be a grey item that has exact amount of sockets, or white item that will end up having exact number of sockets by either using cube recipe to add sockets or lazruk quest.

Blue/yellow/brown/green items are not bases for runewords and they cannot become one by using ang cube recipes