Stellaris

This achievement requires the Plantoids DLC
Fixer Upper may also require the Distant Stars DLC to ensure Ketlings spawn.
Both Fixer Upper and Growing Like Weeds can be unlocked in the same game.

Create a Plantoid or Fungoid empire with the Gestalt Consciousness ethic, Hive Mind authority, the Idyllic Bloom civic, and the Budding trait. Pick any origin story other than the Clone Army.

Start a game with no enemies and focus on expanding your empire and exploring to find the Junk Ratling origin systems (see the wiki for more details if you aren’t familiar with them: https://stellaris.paradoxwikis.com/Unique_systems#Junk_Ratli...). The key system information you need is replicated below:
“A random pulsar system in the galaxy might contain a Tomb World planet called Ruinous Core orbited by a Tomb World moon called Lost Encampment, as well as various debris throughout the system. A similar nearby system will contain three Tomb Worlds: Silent Colony, Decayed Hub and Crumbling Borough. A second nearby system will contain a Tomb World called Fallen Outpost. Unlike other Distant Stars systems, these systems will give a notification when one is entered for the first time.”

When you find the Ketling home systems, settle all five of them within 25 years to prevent the Ketlings from creating an empire that you will then have to defeat. Note that it is possible under some circumstances for the Ketlings to spawn on their own before you find them. While that shouldn’t happen under the game setup as detailed above, if it does, you will need to capture those systems from the Ketling empire to proceed. There is a small chance that your galaxy doesn’t spawn the necessary systems. The only way to tell is to fully explore it, and restart if the systems don’t exist.

Once you have all five planets settled, terraform them to Gaia worlds using the Gaia Seeder buildings. You need to complete all three upgrades to the Gaia Seeder buildings on each planet. Fixer Upper will unlock once the final planet is fully transformed.

(If you are not using a Gestalt Consciousness empire, you can alternately use the World Shaper Ascension Perk which lets you terraform directly to Gaia worlds, but that isn’t available to gestalt empires. That may be a quicker approach if you are not trying to unlock Growing Like Weeds in the same game.)

If you are having issues getting the Junk Ratling systems to spawn, increase the size of the galaxy you are playing in. In the largest galaxy, there is an 80% chance of them spawning.

For Growing Like Weeds, you need a monthly assembly (not growth) of 10. Normally, this requires a population of 500, but with a Hive Mind and the Engineered Evolution Ascension Perk (which unlocks Cloning Pools) you can get there with a population of 150. Terraform your capital planet to a Hive World (this requires the Ascension Perk of the same name, which becomes available after you research Climate Restoration under Society Research) and ensure you have a Spawning Pool (which should have been there at the beginning of the game) and Clone Vats. When your population on your capital reaches 150 (you can resettle from other planets to speed this up) the achievement should unlock. You can view the assembly rate on the Population tab if it doesn’t unlock when expected. I could not get this to unlock on a normal (non-Hive World) planet, and it also would not unlock while I had the Mutagenic Permutation Pool on the planet, even though that contributes to pop assembly.