You don't have to take everything in your closet all at once. Start with favourites or one style or occasion.
Take photos in the day or in a bright room that minimizes shadows on the item.
Find a spot with a plain, uniform, background like a door or a wall.
It works best if it is a different color from the item - so try not to put a white shirt against the white bedsheet.
Hang your items or lay them out flat.
Do up buttons and zippers and smooth out the wrinkles