Heat the oven to 400°F, scrub the potatoes, dry them well, pierce each one several times, wrap each potato in foil, and bake for 1 hour. The potatoes are ready when a fork slides through the center without resistance; let them cool slightly before handling.
Brown and crumble the ground beef in a large skillet over medium-high heat, then pour off any excess grease before stirring in the taco seasoning and the amount of water listed on the packet. Cook until the beef has no pink areas and the seasoning smells fragrant, then bring the mixture to a boil.
Lower the heat and simmer the seasoned beef for 10 minutes while the sauce thickens around the crumbles. The filling should look moist and cohesive rather than greasy or watery.
Unwrap the slightly cooled potatoes, cut a slit across the top of each one, and gently press the ends to open the center. The flesh should separate into soft, fluffy flakes that can hold the filling.
Spoon the taco beef into each potato and finish with shredded lettuce, cheddar cheese, sour cream, salsa, guacamole, and green onion. Add the cold toppings at the end so their crunch, creaminess, and freshness stay distinct.