Instructions
Open your bag or bags using the prompt, "Shift+B." Sell or store any nonessential items from your bag to free space. Open your character menu using the hot key "C."
Left click and hold an item of your character's clothing; a little icon of the item will appear. Drag the item to an empty slot in your bag and release.
Repeat the left click and drag for all remaining items of clothing. Close the character menu and bag using the same prompts or by clicking the "X" at the top of the menu.