When it comes to configuring buildpacks, the order can matter, so you need to consider the order of the buildpacks in the app management screen.
In app management screen, you can remove, add or reorder the available buildpacks.
In the above example, you can observe that only the heroku/ruby buildpack has been added. To add another buildpack, you can click on the "Add Buildpack" button. Which will open the following modal:
You can select from the available list of officially supported buildpacks or you can enter a publicly available Github URL of a buildpack.
To reorder buildpacks, you can simply drag and reorder buildpacks from the same section.
Adding, removing or reordering buildpacks need a full redeployment for the changes to take effect.