Initial note: This achievement will come through natural gameplay, and does not need to be forced in any way, as long as you have a "Fuel Scoop" equipped. This solution is aimed at players who want to get this achievement as fast as possible.
In order to successfully scoop fuel, you first need to add a Fuel Scoop to your ship's loadout.
As a first step, fly to any starport that has a "Outfitting" facility. You can check if a starport has one by looking at the system map and selecting the starport; it's available facilities will be listed in the "Info" tab.
Once in the starport, select "Outfitting" in the menu and then swith from the "Hardpoints" to the "Internal" tab. Select any internal compartment (ideally one that is empty), and buy the largest Fuel Scoop that you can afford - the cheapest one, Fuel Scoop 1E, will cost you only 310 credits; however, it will take you a very long time of scooping to reach the 100 tons required for this achievement.
As a next step, open your Galaxy Map. Select the third tab "View", followed by selecting "Star Class" within the "Filters" - "Show by colour" pulldown menu. Uncheck all boxes except the first seven listed, i.e. O-, B-, A-, F-, G,- K-, and M-Type stars. These seven represent the scoopable star classes, while all others are non-scoopable. Now use the Galaxy Map to plan a journey to a star of a scoopable class, ideally in a distance that depletes your fuel tank; this will be the last system that you could reach in a route before the route line becomes dashed.
Hyperjump to that system, and as soon as you leave hyperspace throttle down to zero. Face the star, then slowly increase the throttle until your fuel scoop starts working automatically. Decrease throttle to zero as soon as you have reached about 80% of your fuel scoops' maximum scooping capacity (Fuel Scoop 1E will be able to scoop 18 kg/s, so you would be aiming at ~14 kg/s). Now sit and wait until your tank is full, fly to another system to deplete your tank again, then rinse and repeat. The achievement will pop as soon as you have scooped 100 Tons worth of fuel.
Things to note:
When you get too close to a star, your ship will suffer heat damage. For a Sidewinder, heat levels of about 100% are still ok, but if the heat continues to increase you should immediately back off and increase your distance to the star. These heat levels and related effects differ depending on the ship you use, so you might need some trial and error.
While you hover and scoop pretty much motionless in front of a sun, you are very vulnerable to enemy interdictions. If you get interdicted at such low speeds, you will submit immediately and have no chance to escape.