In order to start modifying the game, you will need to first and foremost download the Mod Development Kit. Just as you prepare to download the editor, take note of the directory you will install it to (e.g. "C:\Program Files\Epic Games"); it will be necessary for later.

Once the MDK is installed, navigate to the install directory of the MDK, then the Game folder and click on the "WedgieSimulator.uproject" file to properly launch the editor. (Should look similar to "C:\Program Files\Epic Games\WedgieSimulatorMDK\Game")
If it asks for an engine build to use, just select the mod editor build and continue.
It is important to note that due to the frankly TERRIBLE guidelines and conditions required by Epic Games' End-User License Agreement, the editor setup in the Game Store is botched. Do not use the editor shortcut that may have been generated to access the editor. It will lead to annoying errors that has broken proper access to the engine. For now, Underwear Demesne is working hard to resolve that mess... Instead just use the uproject file as instructed earlier.
When the editor has been launched and is ready to use, you'll want to click on this icon on the tool bar at the top center of the editor viewport.

This will launch a prompt allowing you to choose what type of UGC package (AKA mod) you wish to create.

Left-click on a template to choose it (or choose more by first holding down the Ctrl key, or cascade-select by holding down the Shift key), type the name of your package along with the author (could be your name or the name of your team) and description of what your mod will provide. When you are ready, click the "Create Mod" button.
This is where your mod will appear when it is created based on the name you've assigned to it. They are essentially special plugins. You should strictly place all of your content inside that plugin folder.

Additionally, you can read the following tutorial wiki pages to guide you on how to create custom content for Wedgie Simulator:
Developer Wiki: Accessories
Developer Wiki: Eyewear
Developer Wiki: HUDs
Developer Wiki: Headwear
Developer Wiki: Home
Developer Wiki: Mannequins
Developer Wiki: Menus
Developer Wiki: Playgrounds
Developer Wiki: Possessable Mannequins
Developer Wiki: Props
Developer Wiki: Underwear
Developer Wiki: Vehicles