You can promote it the same way as every other app :)
Here are the most importan aspects of app promotiont:
1) ASO – app store optimization. That's very similar to SEO but narrows down basically to App Store by Apple and Google Play.
The first aspect here is improving findability of your app by researching, analyzing and selecting proper keywords.
The second is improving the very page of your app. That includes an icon, screenshots and information. Also, having positive reviews and high rating is crucial for raising the number of installs. Also, AppStore and GooglePlay consider ratings in ranking apps, which is why reviews may come useful too. Today buying reviews is getting more and more popular as a way of promotion. Our service FewReview may help you with that!
2) One of the most optimal way is just simply buying traffic. Google AdWords, as well as advertising on Facebook and other social media may make your number of downloads rocket.
3) Moreover, there are many sites that specialize in writing reviews for apps. Check out 148apps.com, apps4review.com, TheiPhoneAppReview.com, androidapps.comand many many others. You can try to buy an article about your app here. Paid articles might be too expensive for a small developer, however some sites even make reviews for free, because it's the content their sites actually thrive on.
4) Engage your audience! Start your pages on Facebook and other social networks and communicate to your potential users via it. Integrate your app with social networks is also efficient. Remember: word-of-mouth is the best type of advertising!
I hope you find my answer useful!