You don't need to tell the reader what they can skip; instead, you should tell the reader what is in each section and how the sections relate, and they will decide for themselves which sections to read.
The truth is that very few experienced researchers actually read a paper end to end like a novel. Instead, a paper is typically subjected to a sort of literary triage, in which the reader first skims the paper and hits the high points (e.g., title, abstract, figures, theorems, conclusions) in order to determine whether the article is relevant enough and credible enough to invest more time in reading. Even after that, readers will often skip around, deciding which bits are worth investing in reading thoroughly, and which to come back to later if needed. See, for example, this nice manual on how to read a paper from a course at Rice University.
Some paper formats are effectively set up to enable such reading. For example, many high-impact journals like Science and Nature have an ultra-short article format, in which the whole article functions as an extended abstract, and most of the details are typically relegated to the supporting information.
For long format documents, however, such as many other journals, or book chapters or theses, you instead want to be explicit about the structure of the document, saying clearly what the purpose of each section is in the introduction, and what the subsections are for at the beginning of a section. For something that you are reviewing or reproducing for reader convenience, just say so, e.g.:
Section II.A reviews the measurement methodology that we have adapted from [cite].
and the reader will have enough information to know whether they want to read or skip.