Make a new file, the name is important, it must be camel-case and not a .ts file (the @ts-check declaration at the top of each file uses JSDoc to check types while developing rules – read here for more info). The suffix must be ...Rule.js.

You will need to convert your camelCase name to kebab-case and add it to the tslint.json

E.g. noDoingAnythingRule.js -> no-doing-anything and:

{
  "extends": ["tslint:recommended", "tslint-react", "tslint-config-prettier"],
  "rules": {
    "no-import-un-mocked": true,
+    "no-doing-anything": true,
    "curly": false,
    ...

You want to export a class called Rule that lints your file, this is what it used on the file.

Ideally you can do simple string matching like in noImportUnMockedRule.js - but if not, study how to make an AST walker here