You have two basic ways of transition from ikkyo to nikkyo. One is done with thumb leading the rest of your hand around the wrist of attacker. The other one is with the palm of your hand sliding around the top of the attacker hand. In both cases you preserve the very close contact. It is rather difficult to describe with words. But more important part is to focus on complete control on the attacker body with your weight while you do this transition.
Some people do not like the first method "thumb leading rest of your hand around the wrist of attacker" because it's easier to lose the grip on uke, thus allowing uke to escape... what do you think?