To set up a Paid Partnership collaboration in Instagram you can either send a request to a Brand that you are working with or, if they've already sent you a request, you can accept a request from a Brand.
Both can be accomplished by opening Instagram and Navigating to your profile by tapping on your avatar in the bottom right of the screen.
Next, click the 3 bars icon in the top right and choose settings.
Tap on Creator from the Settings Menu.
Tap on Branded Content Ads.
Tap Brand Partners to either accept an invitation, or search for a brand to send them a collaboration request.
Once a request has been sent and accepted, either by the brand to you or from yourself to the brand, you will be able to post paid collaborations with that brand to your Instagram.