How to edge a lawn?

Answer 1

If you are referring to constructing an edge around the lawn with a permanent material, you can use brick, wood or synthetic bender board.

https://www.gardeningknowhow.com/lawn-care/lgen/tips-for-choosing-lawn-edging.htm

If you are referring to cutting the edge:

There are various methods possible, by hand and with power equipment.

https://www.gardeningknowhow.com/garden-how-to/tools/using-a-garden-bed-edger.htm