David is spot on - do as much as you can in the lathe, getting the fore and aft parts of the flare to the correct profile. Then file the right and left sides to the correct shape and matching (not as straight lines shown in the second photo, that's only diagrammatic !) and then blend the fore/aft and right/left curves together.
While you're set up for turning its sometimes useful to do a spare just in case you don't get the filing right first time (hence the spare unfinished chimney)
Edited by PGH, 15 June 2018 - 16:33 .