Without guidance? Extremely hard. I've had quite a lot of brilliant BSc students (or recent BSc grads) as interns who were able to get quite a lot of work done. The problem is, writing a paper is a lot more than simply doing the experiments. Primarily, I found that many students at that level struggle writing cohesive paragraphs, describing how the literature relates to the work that's being done, and clearly explaining the concepts, experiments etc. Then, you typically get weak rejects because the writing is not good or things are not clear.

Many people make the assumption that publishing is about ideas. This couldn't be more false. In fact, execution is often more important than the idea itself. A poorly executed good idea will get rejected. Well executed average idea will be accepted. Apart from these, as I said above, writing is one of the primary elements of the paper. I have seen quite a lot of papers where decent writing basically carried the entire paper, even though the idea was meh.

All in all, when you publish for 5+ years, you generally take many things for granted. But, if your are writing a paper for the first time without any guidance, you are looking to learn things the hard way (via rejections).