There are several ways to obtain a bibliography style. Which one you should take depends on your situation.
Make your own .bst with makebst. This tool is available in MikTeX and TeX live and runs under Windows, Linux and OS X. You will answer a series of questions and in the end you will get a .bst file that conforms to the answers you gave. If you have to change something really unusual, this tool might not be able to help you. Plus there really is quite a number of questions. See also Is there an (easy) way to create or personalize .bst files?
Modify an existing .bst file. Of course you should only modify a renamed copy of an existing .bst. Depending on how much you have to change this is a viable option. But for me at least the Polish notation of the .bst makes it cumbersome to modify.
Find a biblatex style and modify that. biblatex styles use a more LaTeX-like syntax and are therefore easier to modify than .bst files. You can either take one of the standard styles as basis, or use one of the many available custom styles. Be aware though that custom styles can be much harder to modify since some of them have to go through great lengths to get the right output. Most things that 'should be easy' are actually easy with biblatex, but there are the odd changes that should be easy yet are not easy at all. Additionally, biblatex is still under development and 'new', so it might not be in everybody's workflow yet and version conflicts (between collaborators or machines) can be a problem.
Write a new biblatex style from scratch. At least for me this comes down to 3 since I always need inspiration and guidance for such things.
If all else fails, there is always the manual thebibliography way.
In 2. and 3. it is important to find a list of available styles and their output, so you can determine where to start from. For .bst files there is Where can I find collections of bibliography styles?. For biblatex I would use the CTAN topic site (https://www.ctan.org/topic/biblatex) most styles come with example files that show the output.
Obviously in an ideal world you would just use the method that mean the least work for you.
There are a few caveats: Very few journals can accept biblatex submissions. Biblatex: submitting to a journal, there might be tricks to get things going, but in general the outlook is bleak.
Some journals might not accept external files, so with them you can't bundle your custom .bst files.
In your situation where you have to hand in a Word file, I would seriously consider modifying the bibliography manually in Word after conversion from TeX. You will have to fine-tune and check the converted document anyway. This is a good option especially if you bibliography is not too large and the modifications modest.
To your specific problem: I don't think my answer to Biblatex numeric style: replace square with round brackets in both citations and bibliography is too complicated and I can assure you that it works, but I will admit that it needs a bit more code than one might have thought. (It's one of these 'that should be easy' things...)
With biblatex-science, you are lucky that the developer used a slightly more radical method. There getting square brackets back (i.e. reversing his changes) is as easy as
\makeatletter
\renewcommand*{\bibleftbracket}{\blx@postpunct[}
\renewcommand*{\bibrightbracket}{\blx@postpunct]\midsentence}
\makeatother
MWE
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[british]{babel}
\usepackage{lmodern}
\usepackage{csquotes}
\usepackage[backend=biber, style=science]{biblatex}
\bibliography{biblatex-examples.bib}
\makeatletter
\renewcommand*{\bibleftbracket}{\blx@postpunct[}
\renewcommand*{\bibrightbracket}{\blx@postpunct]\midsentence}
\makeatother
\begin{document}
This is a test to cite \autocite{sigfridsson} and cite \autocite{worman} and cite \autocite{sigfridsson}.
\printbibliography
\end{document}
biblatex-sciencestyle really is fine in all other respects, it's trivial to change the parentheses, as the linked question. The other answer (usingmakebst) will produce a file usable withnatbibnotbiblatex, but is also a possibility. I assume the journal doesn't want LaTeX source, but PDF? – Alan Munn Jul 16 '17 at 04:21makebstis actually a TeX program, although it will also run under LaTeX, sotex makebstorlatex makebstshould work in any OS. The only disadvantage ofmakebstis that you must answer a lot of question fixing every style aspect, to the point that modifybiblatex-sciencestyle probably will be easier and simpler. – Fran Jul 16 '17 at 07:38