If a samurai was really unsheathing it they'd hang on to the scabbard. They'd do it regardless or else it'd tug on their clothes and wiggle around and just look awkward/impractical. So imagine them grabbing the sheathe, bending it slightly so it's fully vertical and then unsheathing it (with their knuckles facing behind them when grabbing the hilt). Then as they unsheathe it, it will be in a practical cutting motion with the sharp edge slicing upwards and then forward.

I sound like an edgelord typing this but that's how it's suppose to work I imagine.