1. Rename every file from *.js to *.ts
2. Set the compiler options

   {
     "checkJs": false,
     "allowJs": true,
     "noEmitOnError": false, // so the compiler compiles code it can’t prove right yet. Reset this after you’re done migrating
   }
   

3. Install type packages for dependencies that don’t bring type information out of the box, for instance

   npm i -D @types/d3
   

4. Add // @ts-nocheck to the beginning of every file.
5. Go through your project file by file, remove the comment from (4) and add types until the errors are gone. And probably fix some errors along the way.

Abbreviated from “TypeScript Cookbook” by Stefan Baumgartner.