As uke raises hand for yokomenuchi, quickly enter towards the striking hand before it gets past uke's center. Block high. Duck under and pivot. Should put you right behind them for ikkyo.
Footwork would be, with both of you with right foot forward:
1) Uke strikes with right hand
2) Nage steps with left foot forward to block
3) Nage steps with right foot forward, under uke's arm
4) Nage turns on feet and brings left foot back