I once wrote a paper about using “mutant algae” to terraform Venus. (Later I learned that Carl Sagan had a similar idea in college.) Venus has a problem you see because of waaaay too much CO2. In fact, the runaway greenhouse effect can be attributed to a 96% CO2 atmosphere (with 4% nitrogen.). My proposal was to introduce a biological process that would convert the CO2 to Oxygen by the miracle of photosynthesis (using special high atmosphere mutant algae.) You simply combine some sunshine, a little water with the CO2 and you get a formula something like this 6CO2 + 6H2O ——> C6H12O6 + 6O2.
In effect you are converting CO2 to Oxygen and viola … you get massive cooling (if done on a large enough scale.)
OK why won’t the “mutant algae” theory work.? See those atmosphere numbers up there (96% CO2 and 4% nitrogen) … there is no water. In fact, they is virually nothing on Venus with any hydrogen in it. This means that at mass expense you would have to start crashing water bearing asteroids/meteors into the planet before you could even contemplate changing the atmosphere of Venus. My paper was a load of bullshit sadly (albeit it was written before the atmosphere composition was known.) So I regret to report that without incredibly expensive asteroid moving technology, the goddess of love (apologies to the Botticelli version) shall remain a hot barren world where only Dante’s demons would feel at home. Terraforming Venus is a NO GO.
As a postscript, I note that you might be able to orbit the planet high above the killing pressure and heat and rotate your habitat to simulate a day. This would not exactly be terraforming, but you could claim to be a Venusian. Alternatively, you might be able to create artificial people that could live and work on the planet. The pressures and heat are so ridiculous that even this might not work. I guess the old adage about Love being a bitch might just apply to the planet named for the goddess of love.
Terraforming Mars on the other hand … this is a tasty topic indeed.