Most researchers consider citing references within text correctly and appending a matching list of references at the end of a research paper as tedious chores. A recent study  focused on erroneous references in a single journal and found errors in about one-fifth of the references.
In text, references, or sources of information, are cited either by numbers (sometimes referred to as the Vancouver method) or by names of authors and the year of publication (the Harvard method). This post is about a few minor matters of details of citing references by numbers.
Although citing references by numbers appears to be self-explanatory, there are a few matters of detail, which are listed below. Examine your target journal and format the citations to match the way they are printed in the target journal.
Position of the number (superscript or normal). Check whether the journal uses superscripts for numbered citations. Some journals use superscripts; others print the numbers normally (“in line” numbers).
Surrounding punctuation. Typically, when citation numbers are printed normally, they are enclosed in square brackets (as in this blog post) or in round brackets or parentheses (EndNote, a software package to handle references, typically uses such round brackets). Superscript numbers, on the other hand, are not so enclosed.
Placement. Some journals print citation numbers immediately after the full stop; some journals place them immediately before it. Similarly, when citation numbers appear within a sentence, they are usually placed before a colon or a semicolon but after a comma.
Multiple citations. Separate multiple citations, so long as they are not consecutive, with commas; do not insert a space after a comma (2,5,9 and not 2, 5, 9). Separate two consecutive numbers with an unspaced comma (7,8 and not 7, 8) but indicate a range of consecutive numbers with the en dash (2,5,9–11).
 Mertens S and Baethge C. 2011. The virtues of correct citation: careful referencing is important but is often neglected even in peer reviewed journals. Deutsches Ärzeblatt International 33: 550–552 [Deutsches Ärzeblatt International is the official journal of the German Medical Association.]
["Publish and prosper" is a series of posts about tips for researchers whose first language is not English but who submit papers to journals published in English. The series touches upon not only writing (spelling, grammar, punctuation, usage, and style) but everything else relevant to publishing research papers that journal editors wish their authors knew.]