Hi,
I don't have time to go into detail but here's a few tips:
install your scripts in the Custom Content Folder. You can access that from the file Menu. If you haven't set it yet, you should do it when prompted. Once you have this folder open, you can install scripts in the Script folder. Note that tool scripts go in the Tools folder, Menu scripts in the Menu folder, etc.
The documentation that comes with the script should explain what kind of script you have. if there is no documentation, there is probably information inside the script file itself. You can use a regular text reader to read it.
The Custom Content Folder is the place where you can add modifications to Moho without disturbing the original program data files. The way it works is that whatever you add here will
non-destructively override the file normally loaded from the original program folder, and whatever doesn't exist in the original program folders gets added from this area. While you can install scripts and mods directly to the original program folder, that's generally a bad idea. If you run into a bad script or mod, It's a lot easier to troubleshoot problems when you've placed the items in the CCF. Also, if you update Moho, using the CCF keeps Moho from overwriting your custom scripts and mods.
To update Moho with changes you make to the CCF, you need to quit Moho and restart it, or you can press CTRL+SHIFT+ALT+L to force it to reload from the folders. This shortcut works for Moho 12.x and earlier. For Moho 13 you can CTRL+SHIFT+ALT+R.
Note that some tool scripts require you to also install the button icons for the GUI. Menu scripts don't need this, and you can nest menu scripts inside folders to get cascading menus. I like to add a category folder called '+Special' and add my custom scripts here, further categorized by author menus. (The '+' places this menu at the top of the list for quicker access.)
There's info you might glean from watching some of my videos here:
https://littlegreendog.com/movies/tutorials/
I would look at the one on Layer Shortcuts. It's not specifically about installing scripts but it demonstrates how to install a specific script. In general, most scripts are installed this way.
As for writing Moho scripts, I'm not much help there, but try dissected existing scripts and play around with modifying the lines and see what they do. I haven't written anything significant for Moho but that's how I learned to write a few tools I use in After Effects. (I'm not really a scripter, I just dabble when I have time, which is almost never.)
There are several expert Moho scripters here who can offer more useful advice. Hopefully, they'll pop in post some info soon.