If you’ve already got at least one other Custom Field in the project, then follow the instructions below.
If not, then add any other field, then follow the instructions below, then delete that other field if you don’t need it. Or ignore all that and just Browse the custom fields library and add Estimated Time and then add Actual Time.
Click Customize (button in top-right header), then Fields, then find it here:
Sorry, I’ve updated the previous post; the first custom field you add is a special case, and it seems it’s no longer straightforward to find at that point.