Here's how it works:
Open Instacommentor (you'll have to add you Instagram account to use the app). Click on My Posts and wait for it to refresh.
When you get to the next screen click on the orange arrows in the top right corner. You'll then see a list of posts that have unanswered comments. On the first image shown I have 5 Not replied and on the second image there are 3.
Click the number under "Not replied" and you'll be taken to another screen where you can either answer the comments individually or as a group (up to five usernames per reply). In the example below, click on the Reply balloon to the right to compose your response.
Type in your reply and hit Send. Your Instagram account will be automatically updated! Every once in a while the app gets a little buggy and you end up replying twice, but hey, Mom would be the first to say you can't be too polite!
So, go make your Mom proud and be a polite Instagrammer - next on your list, how about helping an elderly person cross the road?