If you need to import a package without types available, first you should try to install @types/foo to see if that exists. If it doesn't or it's unmaintained, you can create your own typings here.

To add new typings for a package, create a folder with the package name and place a file called index.d.ts inside of it. Any types your export there will be used whenever you try to import the package elsewhere.

In general, avoid adding types here that are simply shared between packages. It's better to have the types owned by a particular package and then import them into other packages. If you only rely on a package for its types you can always add it as a devDependency only.