Here is the way I do it, and I recommend everybody else doing it the same way
Step 1: If you don't have a second character, or have one you don't use, use your second character. This hides your money stats, as well as makes this glitch easier, as you will have more garage space. Purchase two garages, and fill them each with 9 Elegys. I recommend getting the Murrietta Heights as one of your garages, as it has a very short drive out/in time, and it's literally a 10 second drive to Los Santos Customs.
Step 2: Buy ONE Adder from Legendary Motorsport.
Step 3: You DO NOT have to have a Journey blocking the garage. I don't know why this is even a step for the glitch, I made a complete dupe in both my Murrietta Heights garage, and Eclipse Towers without the Journey, the cars can still be sold.
Step 4: If you can't find a hooker next to Eclipse towers (I had trouble with this), drive to the strip club at night. There is always a hooker waiting there for you.
Step 5: Get the hooker in your car, and shoot your gun to scare the sh*t out of that bitch.
Step 6: Find a narrow alleyway that blocks her way of getting out.
Step 7: Get out, and blow her brains out with your auto shotty (or whatever weapon of choice).
Step 8: Drive her into one of your garages, get out, and select the vehicle management menu. Select the Adder that she is in, and replace it with the Elegy.
Step 9: Drive out, and then repeat Step 8 a total of 8 more times.
Step 10: Once done, take the original Adder that the hooker should still be in, and drive it to your second garage.
Step 11: Repeat Steps 8, and 9, until you have TWO garages filled with 9 Adders each.
Step 12: Sell all 18 of your Adders, and save your original Adder that you bought so you can continue to do this process again and again until you have the amount of money you're happy with!
1. You need to make sure you have enough money in the bank to purchase an Adder before even bothering to make a second character.
2. If making a new character, you will not have any upgrades unlocked besides DLC upgrades, but upgrade it the most you can. It's okay, your Adder won't sell for FULL price, but it will still sell for about $630,000.