The ability to schedule your post to send in the future is a Ripl Pro feature. After you've customized your post, you'll be on the final "Caption Your Post" screen where you can add your caption, relevant hashtags, and select where you want to share your creation.
Tapping "Schedule Post" will allow you to pick a date and time for your post to share. The final "share" button should change to "schedule" once you've successfully scheduled your post.
Note: Instagram, does not allow apps to post on your behalf. Any posts to Facebook or Twitter are posted on your behalf when you schedule them. However, if you schedule a post to go to Instagram, we will send you a notification at your scheduled time to remind you to complete the post. If you have notifications turned off, you won't see this prompt!
We copy the caption you entered in Ripl, so simply tap the notification to open Ripl, then "launch Instagram" and you'll be taken directly to the posting screen in Instagram. Double tap to paste your caption and complete your post. If you manage multiple Instagram accounts, be sure you're in the correct account prior to posting.
For more information on Ripl Pro, check out this article: What is Ripl Pro?