How do you get the sandman in tf2?

The Sandman is an achievement unlock (Milestone 2) but you can also get the Sandman from random drops, you can uncrate it (Strange Sandman Crate #23) it can be gifted to you, you can trade for it, you can buy it from the Mann Store (or another player) you can also craft it.

Blueprint: Class Token - Scout + Slot Token - Melee + Scrap Metal

NOTE: This does not guarantee a Sandman, but one of the nine possible melee weapons for scout, Sandman being one of them.