See Spines in 3D and the tutorial (PDF) for step-by-step instructions
To see all the options, select spines first.
OR
You can't undo this action.
You can only use the Spines toolbar in the classic window, not in 3D.
To combine two spines, use the Merge button.
You can only merge two spines at a time; three or more simultaneously is not possible.
To separate two spines that are modeled as one spine due to physical proximity, use the Split button.
In most cases, the split location is unambiguous and only one split operation is required to obtain the two desired spines. For more complex cases, you may need to perform several Split and Merge operations to obtain a satisfactory model.