From within a campaign, click "Settings" in the bottom left of your screen.

Click the Custom Metrics in the setting page menu, then create custom metric in the top right. It may also show in the middle of the screen, if no custom metric has been created for this campaign.

You will be presented with a screen asking if you wish to create a new custom metric or clone an already existing one. Choose "Clone an existing custom metric".

Next, you will be asked to choose which custom metric you would like to clone. When cloning, you can choose the campaign that contains the custom metric you would like to clone from. Use the checkbox to indicate the custom metric to clone, then click "Select custom metric".

Next, you will be able to edit your custom metric if needed. Note that these edits will only apply to the clone you've just created. You can change the name, data type, description, and formula of the custom metric.

When you're done, click "Create" and your cloned custom metric will be complete. You will now be able to view it on the Custom Metrics main page, and add it to any report or custom dashboard via the "add widget" process.
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Note: If you see "an error occurred" after saving your widget, please check that your custom formula is valid and that the integrations relating to your custom formula are connected properly.