How to make a chimney lead back gutter on a bench

How to make a back gutter (cricket) on a bench, step by step, full comprehensive guide and the most common reason for them not fitting.
How to weld lead: https://youtu.be/rGnD7kaDBEg
Content
0.00 - Intro
0:07 - How to measure up & most common mistake
2:34 - Setting out
3:21 - Where to cut
4:12 - Forming
6:33 - Welding
6:58 - Outro