IG does not allow other apps to post directly on your behalf. The best you can do is get a reminder Buffer is the best choice in doing that and a great all around social media scheduler and analytics tracker. Hopefully one day IG will change their API to allow direct posting from other apps, but until then go with Buffer:
I take photos with my iPhone or my DSLR, edit on the phone with Snapseed, do some more specific edits if need be with Facetune, sometimes I use PicMonkey as well. Then I upload with instagram, or use Buffer to start the upload and remind me when its time to post. I use Buffer for all my social posting
I have been using this for quite some time and I love it. I like their calendar view scheduling, the media library and option to repost. They have a free tier which would let you post up to 30 IG posts (+ a number of Twitter, FB and Pinterest posts too) for free every month. Please note that like most of the other IG schedulers, Later as well can't post to IG on your behalf. It will send you a notification to your mobile app at the scheduled time and you have to manually post it. There are a very few (1 o2) schedulers which would auto post without manual intervention. I haven't tested those though. This has to do something with the way IG has exposed their APIs I believe.