This might help....
taken from Bibleprobe.org....
It is saying here that those that make themselves ready to become members of Jehovahs family are blessed and admitted through the symbolic gates to the Kingdom of God. Again, this had nothing to do with the act of salvation by Christ in bringing about the ransom of mankind with the sacrifice of His life. However, it is clear that the individual has to wash his own robes by his faithful action of following the commands of Christ. The individual cannot make these symbolic robes white. That has been done for him. Nonetheless, the individual does have to wash them. Those that do not follow the commands of Christ are equated with those that are outside.
Your right, the NKJV is a little easier to understand, I have the "Nelsons NKJV Study Bible" although I do use the NWT more than I do the NKJV.