I need a step by step guide to making a bbl file from bib

Suppose that we have the following document:

\documentclass{article}
\begin{document}
\cite{article-crossref} and \cite{article-full}.
\bibliographystyle{plain}
\bibliography{xampl}
\end{document}

Now, we compile in the usual way, using our editor of choice or on the command line or whatever. We make sure that everything is properly resolved and defined, running the compilation sequence multiple times if needed. (Compilation is just running e.g. pdflatex -> bibtex -> pdflatex -> pdflatex.)

Now, to prepare the file for journal submission, we open the .bbl file, which will be in the working directory, by default, or wherever you've asked for generated files to go.

Here's mine:

\newcommand{\noopsort}[1]{} \newcommand{\printfirst}[2]{#1}
  \newcommand{\singleletter}[1]{#1} \newcommand{\switchargs}[2]{#2#1}
\begin{thebibliography}{1}

\bibitem{article-crossref}
L[eslie]~A. Aamport.
\newblock The gnats and gnus document preparation system.
\newblock {\em \mbox{G-Animal's} Journal}, 41(7):73+, July 1986.
\newblock This is a cross-referencing ARTICLE entry.

\bibitem{article-full}
L[eslie]~A. Aamport.
\newblock The gnats and gnus document preparation system.
\newblock {\em \mbox{G-Animal's} Journal}, 41(7):73+, July 1986.
\newblock This is a full ARTICLE entry.

\end{thebibliography}

We copy this into our original .tex file at the point where we issued the \bibliographystyle and bibliography commands:

\documentclass{article}
\begin{document}
\cite{article-crossref} and \cite{article-full}.
\newcommand{\noopsort}[1]{} \newcommand{\printfirst}[2]{#1}
  \newcommand{\singleletter}[1]{#1} \newcommand{\switchargs}[2]{#2#1}
\begin{thebibliography}{1}

\bibitem{article-crossref}
L[eslie]~A. Aamport.
\newblock The gnats and gnus document preparation system.
\newblock {\em \mbox{G-Animal's} Journal}, 41(7):73+, July 1986.
\newblock This is a cross-referencing ARTICLE entry.

\bibitem{article-full}
L[eslie]~A. Aamport.
\newblock The gnats and gnus document preparation system.
\newblock {\em \mbox{G-Animal's} Journal}, 41(7):73+, July 1986.
\newblock This is a full ARTICLE entry.

\end{thebibliography}
\end{document}

Now clean to remove the generated files, so you are sure you aren't still relying on the external .bbl and re-compile at least twice (pdflatex -> pdflatex) to obtain the same result from just your .tex file.