When one first reads these verses, it seems strange that a virtuous woman would suggest what Naomi advised Ruth to do. However, it's based on the law (Deuteronomy 25:5).
Ruth had been working hard in the fields gathering grain. Now that the harvest time was over, Naomi, knew that Ruth needed a home where she would be cared for, and not have to work in the fields. Ruth had been working in the fields belonging to Boaz. And since Boaz was a kinsman, Naomi thought it was the right thing that Ruth present herself to Boaz.
Boaz even though he was wealthy and had field of grain, he worked himself at the winnowing the barley. Winnowing is removing the chaff, usually by allowing the wind to blow through the harvested grain. The chat which is light will blow away, while the heavier falls to the ground.
Naomi, then tells Ruth to make herself presentable. Some commentators suggest that she not wear her mourning clothes (from losing her husband); other suggest that she simply put on her "Sunday best" rather than the clothes she wore to work in the fields. Either way, she was to make herself presentable to Boaz.
Ruth was not to run into the place where Boaz was, nor was she to make a scene. Rather, she was to privately wait and keep herself from being known. Ruth remained humble and followed Naomi's instruction.
Ruth's character did not change in these verses. Ruth remained obedient, humble, and willing to listen to Naomi's advice. Ruth trusted Naomi and Naomi knew that Ruth would do what she advised.
Advice. It can be a good thing. But coming from the wrong people with the wrong motive, it can be dangerous. It's key that the source of the advise have wisdom and integrity. It is also key that when the advice is solid and godly advise, that all of it be followed. You don't just follow the parts you like, it all must be obeyed.
How does one know if advise is godly? Compare it to God's Word. If the advice goes against what God says, then it's NOT good advice. Also, consider the source of the advice. Has that person been trustworthy in the past? Is that person one of integrity?
With a heart permeated with a deep sense of reverence for God (Acts 9:31 MSG), it's easier to discern when advise is good. And when my heart is ready to accept good advise, it's much easier to follow through on the advice.