USE WITH CAUTION. All Saved Data will be lost!
To format the disk, while booting the game, hold all C-Buttons until you see a red screen with text.
Then press the following buttons in this order:
(C-Up, C-Down, C-Left, C-Right, C-Down, C-Up, C-Right, C-Left, C-Left, C-Right, C-Down, C-Up, C-Right, C-Left, C-Up)
The disk will be formatted afterwards and directly launch the game.
Ramsie is a hidden game that you can play by performing the following:
Go to the Game menu, and point to the Cartridge icon near USER GAME, and then press the following:
(C-Up, C-Down, C-Left, C-Right, C-Down, C-Down, C-Up, C-Up)
You can then click on the Cartridge icon to play Ramsie.
You can find a translated manual of Dezaemon 3D, which is very important here because the editor is very difficult to use, it is robust and you can do a lot of things, but definitely keep that around:
https://gamefaqs.gamespot.com/n64/574507-dezaemon-3d/faqs/79008
Unlike other 64DD expansions, Dezaemon disks contain only stage data and blank system files. When one of these system files is found on the disk drive the game switches into “disk” mode. This unlocks and expands several features:
Except for the disk bootstrap for when the disk is inserted without the cartridge, there is no executable data on the disk; it only contains resources in a proprietary file system format. Due to this formatting, it is not obvious if it would be compatible with RandNet. Other known RandNet titles use the MFS library, providing not only a standard file storage system but also encoding the game IDs each file is usable with.
Source: Zoinkity (via TCRF)