I'm not too sure on this, as I lost my copy of Pokémon Gold some time ago, and I doubt Crystal can do the same thing, but...
1. Make sure that the Pocket Monster(s) you want to clone (as well as any item you want to clone such as rare medicines/TMs are attached to it) are in party.
2. Head to a PC where you can deposit the Pokémon via Bill's Storage System, such as a Pokémon Center.
3. Boot up the said PC and head to Bill's PC.
4. Select Deposit Pokémon and make sure you have enough room to store the Pokémon.
5. Deposit the Pokémon in the PC.
6. Switch to another Box.
7. It will then confirm whether or not you want to save your game. Say yes.
8. When it says 'SAVING. DO NOT TURN OFF THE POWER.' do turn off the Gameboy ASAP.
9. Turn the Gameboy back on, and select your file. Now, if done properly, you not only get the cloned Pokémon on the list, but also the same Pocket Monster(s), as well as any items held to them, in the PC.
That's that. ^^
-Chris R.