Step 1: Use a mon caught on a Gen 8 game, on a gen 8 save
For simplicity sake: Use any above gen 3 save.
If the mon was caught in any games from Gen 3 - 5, reroll PID. (ignores valid PIDIV; if on non-hatch mon, will result in PID mismatch)
If the mon was caught in any game > Gen 5, reroll EC.
Step 2: Make sure the IVs you have will give you the characteristic (the bulbapedia table)
Step 3: Press This (for EC)
or this (for PID)
Once again, as said in step 1, whether you reroll PID or EC depends on the origin of the caught mon.
Step 4: Change to the Stats tab. Check what characteristic it is.
Step 5: If it is not what you want, repeat from step 3.
Alas, you got it!
This is pretty much as simple as I can make it. If you still don't understand, I suggest you don't do it.