Following the docs doesn't give working results.

[crystalthoughts]

> npm i -D @custom-elements-manifest
> custom-elements-manifest analyze

custom-elements-manifest : The term 'custom-elements-manifest' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling 
of the name, or if a path was included, verify that the path is correct and try again.
At line:1 char:1
+ custom-elements-manifest analyze
+ ~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : ObjectNotFound: (custom-elements-manifest:String) [], CommandNotFoundException
    + FullyQualifiedErrorId : CommandNotFoundException

I get the above error.

I would prefix the last command with npx

[alanwsmith]

I put in the pull request linked above to update the commands.

[thescientist13]

Another option is to make it an NPM script (you can name it whatever you want), which should auto-resolve to node modules, e.g.

"scripts": {
  "cem:analayze": "custom-elements-manifest analyze"
}

Then run npm run cem:analyze

(I prefer "local" installs over npm i -g ... personally)