Changing from ikkyo to nikkyo is killing me on the ura version help please.
Personally, I don't like the 'do ikkyo then transition to nikkyo'-approach, unless one is training how to recover from a poorly performed ikkyo by changing it to nikkyo. Focus on the nikkyo from the start.
Secondly, I was taught nikkyo ura with an entry and setup that's different from ikkyo. So for reference (not suggesting here you replace what you're being taught with what I describe here
): small entering step to the back side of uke, one hand grabs forearm of uke other cuts on the wrist; cutting motion is down while performing small tenkan (for the correct position, as Abasan said: centerlines!); keep control of uke with hand on forearm, other hand goes round uke's wrist to take the nikkyo grip while putting uke's hand/wrist against your shoulder.
p.s.: The technique description above was written with a gyaku hanmi shomen uchi attack in mind.