Connect a T-joint valve ( ie split the single cold water outlet into 2 cold water outlets ) then connect the 2 outlets to your mixer tap with soft hoses. If your tap is new, it should come with 2 x soft hose. If it's an existing one and/or you have no idea where the original soft hoses are, go to a hardware shop and buy them. One end is the normal hexagonal shaped head , the other is a smaller round shaped head. Before you buy them you should just take a quick peep at the base of your mixer tap to make sure.
Or a simpler way... call for a general plumber
