In order to unlock this achievement there are two pre-requisites that must be met first;
1) You have progressed through the campaign enough to have a green 'Y' appear on your map and have gone there to meet Yogi and Reggie for the first time. This meeting ties in with a mission for the campaign so I don't believe this is missable.
2) You have visited them a second time and you have completed the mission they give you. You will earn this achievement after finishing;
The Overdose achievement in Far Cry 4 worth 12 pointsDiscover Shangri-La (Campaign only). At this point you are able to purchase upgrades for the Ghale Homestead from the caretaker outside the house. The upgrades you can purchase are as follows;
Water Faucet - 25,000 Rupees - Body armour can be found here
Trading Post - 50,000 Rupees - Allows you to buy ammo/weapons and sell your unwanted items
Exterior Upgrade - 75,000 Rupees - Fully repairs the exterior of the house
Interior Upgrade - 100,000 Rupees - Fully repairs the interior of the house
Moonshine Kit - 125,000 Rupees - Will always have some Molotovs available for you to pick up
Interior Lighting - 150,000 Rupees - Installs lamps to brighten the house up
Tree Decorations - 175,000 Rupees - Sacred tree decorations, money can be found here
Pinfold - 200,000 Rupees - Adds pigs to the Homestead, you can also pick up throwing knives from here
Garden - 300,000 Rupees - Will add a selection of plants, very helpful if you require some leaves to craft syringes with
Buzzer - 400,000 Rupees - Your very own door buzzer!
Shrine - 500,000 Rupees - A small personal shrine, money can be found here
Purchasing any three of the above will earn you the Fixer-Upper achievement.
EDIT[1]: Thanks to
TheDude722 for notifying me that if you have a karma level of 7 or higher you get a 25% discount off each item.