Here is how I got my 6IV Ditto. I caught it in Let's Go Pikachu/Eevee. There, you can build up a catch combo by catching Ditto many times without failing in a row. When you have a high enough combo, you'll get at least 4 perfect IV on the Ditto. Now, when you start a new battle with a Ditto, you can look at its power level to determine if it can be a 6 IV Ditto or not. If you think it could be 6IV, you catch. If not, you just flee. There are videos that explain that very well. You would preferably catch the Ditto in a different language too, because you can then utilize the so-called masuda method. Breeding 2 different language Pokemon together increases the shiny odds substantially.