You will need Contributions Settings permissions to perform this action.
Go to Contributions > Settings in the left-hand menu, then click the Giving Portals tab. Open the giving portal you want to edit by clicking its name, or by clicking the ⋯ 3-dots menu on the row and choosing Edit.
In the Edit Giving Portal dialog, stay on the General tab and scroll down to the Designation(s) section. Click choose designations to open the selection modal on the right, where you can check the designations you want available on this portal. Use check all or uncheck all to bulk-select.
Once you've selected the designations you want, they appear in the Designation(s) section grouped by category.
To reorder, click and drag:
The six-dot handle (⠿) next to a category name to reorder the category itself.
The three-line handle (≡) next to a designation to reorder it within its category.
Once the order looks right, click Save. Changes take effect on the giving portal immediately.