As per Facebook’s official announcement, direct publishing is currently only available for Instagram Business accounts. So you must have an Instagram Business account in order to use this new feature.
Check this to change your current Instagram account(s) into a Business one(s) (or to check if you already have it).
Once you convert your existing account(s) into a Business one(s), connect it with your Facebook page(s). If you don’t know how to do it, you can find help here.
Now log into your account at Social Web Suite, click on the “Social Profiles”, “Add a Social Profile” and add those Facebook pages that are connected with your Instagram account(s).
When you add the page(s), add the Instagram account(s) by clicking on the “Add an Instagram Profile”.
Once you finish adding the account(s) simply go to our composer at the top of the page and send immediately or schedule a message to Instagram.
In order for a message to be sent to Instagram, it must contain an image. It can be an image-only message or you can add hashtags and text. The text and hashtags you add will show up as the first comment on your Instagram account.
Currently, only a single image is available to be shared and only to a feed. Image must be jpg, the maximum size 5mb, and the aspect ratio 1:1 (square) or between 1.91:1 and 4:5. Images outside this aspect ratio may result in an error.
The minimum width recommended is 320 and the maximum 1080.
The ideal size images to be shared on Instagram are 1080px by 1080px; 1080px by 608px and 1080px by 1350px.
The API rate limits for an account are 25 messages within 24hrs.