The Good Life

Main Quest / Story Related.
Progress through a number of Main Quests until you're working on "Onward to the Grungy Old Castle".
Heading to the red circle marker will complete this quest and result in you beginning an Urgent Quest entitled "Three Trials".
The first step of this is to tame and then ride one of the nearby sheep.
Once you've ridden the sheep in step 1/3 of this quest, Achievement Unlocked.

You’ll have to play through a few other missions to get the Route A, B and C missions – I didn’t keep track but I was around 7 or 8 hours into the game (and I got sidetracked a bit). Edit: If you mainline the story missions, you can get to the Route missions in about an hour. You’ll unlock all 3 'Route' missions at once and once unlocked you can choose which order you want to do them.

Spoilers have been avoided to a degree but consider yourself warned.

An Unique Emblem
Go down to at Burley’s Farm. You can spot it on the map to the South of town if you’re not sure where it is. The Route A missions begin after the short scene.

Onward to the Grungy Old Castle
Leave the farm and take the path opposite, heading East. You’ll have to pay (£10) to open a gate but this will lead directly to the marked location. Approach the marked gate and interact with it for a scene.

Three Trials
Check the instructions on the screen then go get yourself a sheep! Change to a dog and approach a sheep (just next to the gate there should now be another open gate leading to a paddock with several sheep to choose from).
Go near one and press to bark. The yellow marker on the sheep will drop a little and the sheep will run away. Chase it and bark again, then repeat. When the sheep’s yellow bar drops enough it will be marked with a blue light, so walk close to it and press to tame it.
You will automatically get on it, so ride back to Samuel at the gate which will be marked.

After the short scene, you’ll unlock this achievement