putting them in the .txt file and loading them in....where inside the loader/player?
Those TXT files are not used directly by the loader/player.
code.txt files must be created externally on your local disk and organized into Character Folders, as described in the
Newbie Guide. When you've completed this task and restarted the game, your character will appear in the roster (alongside SDChan, Asuka, etc). Clicking her portrait will cause her to be loaded -- including any custom settings which were specified in the
code.txt file.
You can, however, copy-paste a charcode string which you find within a forum post. On the "Modding" tab of the in-game menu you'll find the heading "Custom Save Data" with a text box below it. Paste the text into this field and then click the "Load" button. The customization settings will be applied to your current game; the girl's body and clothing will immediately be changed to match the provided customization settings.
Please note that this is a one-time operation; the changes will be overridden and lost whenever you load another character. You can Save the character after clicking the Load button ... but if you're using the Loader then you should probably setup Character Fodlers instead of relying on the game's Save Character mechanism. Character Folders require a bit more learning and practice, but they allow for much greater customization. For example: a vanilla saved character can have a specific breast
size, but a Character Folder could include a
custom set of breasts.