1710s,

Though you may start VISA screening now, the best time I would say is once you pass the NCLEX.

The process from the very top is something like this,

1) Pass the NCLEX and get your license

2) Find a hospital/nursing home that can sponser you a Green Card. Many people get help from commercial agents (lawyers cum placement specialists) in this regard. I think they may charge you around 1500, but you do not have to worry about the Hospital Placement and Green Card Application. However, US governement acceptance of the green card application varies time to time and they are not accepting currently, as of my knowledge.

3) Start preparing for IELTS or TOEFL iBT.
I do not know about the TOEFL, but 6.5 overall and 7 in speaking is required in IELTS acadamic module.

4) Apply for VISA screen

5) Get the required TOEFL/IELTS score

6) The VISA screen certificate is required by the US government before they can issuse the green card.