This is not really missable and just requires you to complete the first few missions you are given once you enter woody's round-up (you are sent here automatically after completing the first stage on the train).
The missions to complete are:
"Destroy the old shack"
"Clean up the town" (break 3 more shacks)
"Give an alien a hat"
"Paint a Building"
"Talk to Stinky Pete"
After talking to Stinky Pete you will be given a pick and told how to collect gold. Mine your first piece of gold and unlock this achievement.