This is located in Campaigns > Edit This Campaign > Order Settings > Custom Fields (Applied to Entire Order).
First, be sure you've set your standard Information Collection Settings. If there are any other questions you'd like to ask your supporters during the transaction or other data you'd like to collect, this is where you can add those questions. Questions added in this area will be asked to all supporters regardless of which action they take in your campaign. If you would like your supporter to only be asked the Custom Field question based on the action they take see Advanced Settings for how to set this up on a tool by tool basis.
In this area, toggle Add Custom Fields to Entire Order to Yes. Select your existing Custom Fields from the drop-down to add them to your campaign. They will be listed in the order in which you add them here, and you can rearrange them by clicking the up- and down-facing arrows to move them up or down.
If you'd like to add a custom question for which you don't already have a Custom Field yet, you can choose Create New Custom Field from the dropdown. This will give you a shortcut to add a new Custom Field and insert it directly into this campaign in a single step.