1️⃣ I will give you a small Pokémon Trainer Central account (mini-PTC) that holds the Pokémon you bought. 
2️⃣ You then need to build friendship (sending gifts) between this account and your main account. 
3️⃣ Interact daily for 30 days to lower the trading cost to 80K Stardust, or 90 days for 20K Stardust. 
🚩 To make it simple, you do not need to change the password. I will never access the account again once it is sold. 
🚩 The account is yours to keep; you do not need to return it.