I am an early career researcher (pre-PhD) and planning to work on research articles before applying to PhD studies. May I ask is there any clear cut method to approach well-established scholars to ask for collaboration? or our options are only limited to the professors we worked previously? Thank you!
Wide circles of discussion and collaboration are a good thing in academic research. If your research is close to that of some distinguished "elder of the tribe" then it is fine to reach out to them.
But, it might be better to go about it a bit indirectly. Introduce yourself with a few words about your research, and the new idea. Ask the person if they are interested in it and (most important) are there others who might be interested enough to collaborate going forward. You aren't directly asking for the professor's help/collaboration, but are seeking to build a circle of interested parties, a research group. It may be that the professor will suggest themselves, but they may also suggest others, even their own students.
That may not be exactly what you want, but in the longer term might be quite valuable. You seek to become part of their circle so that they think of you when new ideas arise. And introducing you to others is less of a commitment on their part and so might be more likely to be treated positively.