The answer of Richard Slater is informative and the answer of Andriy Buday could look less professional. But, in my case, the answer of Andriy Buday was also very important.
I tried to install two packages atom-beautify and prettier-atom by following the answer of Richard Slater and had some problems of not being able to find some modules. It was not only me who had these problems. Consider checking the following links.
The issue of "cannot find module event-kit"
https://github.com/Glavin001/atom-beautify/issues/1734
https://github.com/Glavin001/atom-beautify/issues/1366#issuecomment-269716306
When I decompressed a file (atom-beautify-0.30.3.tar.gz) I received from GitHub respository, I could find out directories like appveyor, docs, and examples. But I could not find out a directory named node_modules which was present when I installed this package atom-beautify using Atom Editor online.
To check if the absence of directory node_modules is the only problem, I went through the following steps.
Start Atom Editor.
Install atom-beautify using Atom Editor online like the answer of Andriy Buday suggests.
Close Atom Editor.
Move atom-beautify directory from ~/.atom/packages (that was %HOMEDIRECTORY%%HOMEPATH%.atom\packages in my case because I used cmd on Windows 10) to somewhere else.
Decompress atom-beautify-0.30.3.tar.gz and move or copy atom-beautify-0.30.3 directory from this decompressed result into %HOMEDIRECTORY%%HOMEPATH%.atom\packages as the answer of Andriy Buday suggests.
Rename directory %HOMEDIRECTORY%%HOMEPATH%.atom\packages\atom-beautify-0.30.3 to %HOMEDIRECTORY%%HOMEPATH%.atom\packages\atom-beautify as the answer of Richard Slater suggests.
Move or copy node_modules directory from the directory moved at step 4 into %HOMEDIRECTORY%%HOMEPATH%.atom\packages\atom-beautify.
Start Atom Editor.
I found that no error message appeared and that package atom-beautify worked properly, thus I am thinking that absence of node_modules directory was the only problem of the file atom-beautify-0.30.3.tar.gz I received from GitHub repository.
I am afraid if it is normal that directory node_modules is not contained in the file atom-beautify-0.30.3.tar.gz downloaded from GitHub repository because of any rules I do not know yet, like placing directories like node_modules somewhere else. If there really are such rules and somebody tells me about such rules by adding an answer or a comment here, I will appreciate it a lot.
I am not sure if it is same with all other packages, but I found that it was same at least with package prettier-atom.
I wish it helps somebody.
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I found why the directory node_modules was not contained in atom-beautify-0.30.3.tar.gz.
I checked answers of the following link.
How can I manually download packages for atom editor and install them (manually)?
Answer by D3181 included a link to a page of http://discuss.atom.io/ (I could get a helpful answer by Alchiadus from the link) and suggested running apm install in the package's directory. If it is necessary to use a file downloaded from GitHub repository like atom-beautify-0.30.3.tar.gz, it is necessary to run apm install in the package's directory before copying or moving into %HOMEDIRECTORY%%HOMEPATH%\.atom\package (~/.atom/package in case of *nix) of the offline computer.
Decompress the file downloaded from GitHub repository like atom-beautify-0.30.3.tar.gz.
Go into the directory like atom-beautify-0.30.3 of the decompressed result.
Run apm install on an online computer. (If the directory of apm.cmd is not in PATH, run {directory of apm.cmd}\apm.cmd install.)
Rename directory like atom-beautify-0.30.3 to the correct name of the package like atom-beautify.
Move directory with the correct name of the package like atom-beautify into %HOMEDIRECTORY%%HOMEPATH%\.atom\packages of the offline computer.
Run Atom Editor on the offline computer and check if the package works properly.
It seems normal that the directory node_modules is not included the the compressed file downloaded from GitHub repository.