Getting the toaster expert achievement involves finding 5 golden toaster parts via using the toaster repair skill on toasters found throughout the game. Max toaster repair skill required is 8. This also leads you to the Verchitin Armor set which is pretty much the best armor for melee characters.

In the remains of the Wesson House you'll find the first toaster to repair for a part.

In the hoon homestead you'll find the second toaster to repair for a part. It is available regardless of who you helped in the timed mission.

After you deal with Valor, the first of the Patriarch's children, you'll get a radio call about some women showing up at Ranger HQ. If you talk to them it starts a side quest that leads you to the Painted Mines Map where you can find the third toaster to repair for a part.

When you're in Aspen dealing with Victory, you'll find the fourth toaster to repair for a part a pretty short ways into where you first enter the map from.

I'm pretty sure you don't actually need this for the toaster achievement, but some people think it's related or necessary. On the interior map of Aspen (inside the lodge after the ride on the lift) you can find a vault with a golden hammer inside of it. It's used to open a secret room at the entrance to the lodge. To get the hammer either have lockpick 10, or kill the servant in a room nearby for the key (he tells you he has it in dialogue)

After the events in Aspen and reporting back to the Patriarch you'll get access to the lv 3 Kodiak Chassis which lets you explore the Eastern Plains area. While there if you get close to the Knox Bison Ranch (northern part of the Eastern Plains area) you'll get a side quest via radio that lets you enter it. (Sometimes it doesn't trigger right away when you first get access to the area, if that happens just do some quests etc and come back to it.) On the ranch after you deal with the side quest you can explore and find the last toaster to repair for a part.

After you have all 5 gold pieces in your inventory, dialogue will automatically pop up assembling the toaster. It has coordinates for a toaster repair academy on it, which unlocks the area on your world map. Head to the academy and interact with the altar there (its a very small one room map) and once you do it'll spring to life as Courageous the toaster. It is immune to damage, deals average burning damage mostly, but sometimes uses a mortar attack that is devastating.

The Verchitin Armor Set is also in the large container nearby.