How to Make an Internal 90 Bend in Electrical Trunking using an Angle Grinder

The how to guide to manufacture an internal 90 degree trunking bend, step by step clearly showing how to make to a set measurement. This method uses an angle grinder although an hacksaw would be also as good (the method I normally use to demonstrate to students in a class safer). Looks as good or better than a bought one. Lid not shown but out of all lid to make, this again is the easiest.