There's no one way. Here's a recipe you can use:

 1. Make a proper app post here on StackApps.  
  See **http://stackapps.com/questions/7/how-to-list-your-application-library-wrapper-script-here**.

 2. Look for posts on [Meta Stack Overflow (MSO)][1] and/or on [Meta Stack Exchange (MSE)][2] where your app might make part of a ***good*** answer.  
  If you find such posts, make *proper* answer(s) to them -- linking both to the app post you made in step 1 and directly to where they can install the app.  
  Be sure your answers meet all the regular criteria of a proper answer and avoid "spammy" behavior.

 3. If you can't find such posts, you can create [a self-answered question][3] on MSO and/or MSE -- **provided that such question and answer are appropriate to the site**.

 4. If you have a blog, Facebook, Twitter etc. You can promote your app there, too.
 

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<sub>PS: You can always pay me large sums of cash for advertising purposes. Results are not guaranteed. YMMV. (^_^)</sub>

  [1]: https://meta.stackoverflow.com/
  [2]: http://meta.stackexchange.com/
  [3]: http://stackoverflow.com/help/self-answer