Example of a typical PostCSS config file (cli flavor). I use this for WordPress theme development.

postcss-conf.json

{
  "use": [
    "postcss-import",
    "postcss-mixins",
    "postcss-nested",
    "postcss-advanced-variables",
    "perfectionist",
    "stylefmt",
    "postcss-cssnext",
    "cssnano"
  ],
  "input": "./css-src/main-src.css",
  "output": "./schnell/css/main.css",
  "watch": true,
  "map": "file",
  "perfectionist": {
    "cascade": false,
    "sourcemap": true
  },
  "cssnano": {
    "discardComments": false,
    "autoprefixer": false,
    "core": false
  }
}

• Line 2: Call all the PostCSS plugins you need for this task in array.
• Line 12: Path of the input file. This is the file, where you write your css code,
• Line 13: Path of the output file. This file you use in your theme.
• Line 14: Option to watch over file changes. If you change (save) the input file, then the task will run.
• Line 16 ff.: Some options for the perfectionist plugin.
• Line 29 ff.: Some options for the cssnano plugin

There are some more options for the task. Read the manual: https://github.com/postcss/postcss-cli