I want to install the Document Class that the journal "Frontiers in Physics" requires. (Note: LyX does not list such a class as 'unavailable' [and, so answers to similar questions may not pertain]. In any event, I tried to follow advice in one of those answers. See below.)
I downloaded the template file via https://www.frontiersin.org/journals/physics#author-guidelines and http://www.frontiersin.org/design/zip/Frontiers_LaTeX_Templates.zip.
I followed (several times) the process that How to Install an Unavailable Document Class for LyX on Windows recommends. Once, I did the following.
I created a folder (which I named Frontiers) under Program Files (x86) > MikTeX 2.9 > tex > latex. (I seemed to need to use Windows ‘Admin privileges’ to do this.)
I extracted all the material from the folder Frontiers_LaTeX_Templates into the folder Program Files (x86) > MikTeX 2.9 > tex > latex > Frontiers.
I ran MikTeX 2.9 > Settings (Admin), and triggered (first) ‘Refresh FNDB’ and (second) ‘Update Formats.’
I ran LyX -> Tools -> Reconfigure.
I closed LyX
I restarted LyX, specified File -> New, and then tried Document -> Settings.
I looked at the drop-down list under ‘Document Class’.
I did not find anything that I could associate with ‘Frontiers’. (I looked – in the drop-down list - especially at ‘Articles’ [including items listed as ‘Unavailable’] and at ‘Uncategorized’.)
On later tries, I added (and then extracted into) the folders … Program Files (x86) > MikTeX 2.9 > tex > Frontiers … and … Program Files (x86) > MikTeX 2.9 > tex > latex > lncs > Frontiers.
For various (including all but the first) attempts, between steps between steps 2 and 3, I tried to do … Windows System -> Command Prompt -> C:\Users\test>texhash. This appeared to report two lines of ‘successful’ activity and two lines of ‘perhaps not successful’ activity. Here is a 'scape' of the last such report.
C:\Users\test>texhash Creating fndb for user root directory (C:\Users\test\AppData\Roaming\MiKTeX\2.9)... Creating fndb for user root directory (C:\Users\test\AppData\Local\MiKTeX\2.9)... Skipping common root directory (C:\ProgramData\MiKTeX\2.9)... Skipping common root directory ("C:\Program Files (x86)\MiKTeX 2.9")...
I would like advice as to how to get this to work successfully.