Skill...scrapers are rather simple tools. Setting the card for your work is the only tricky bit, and videos on the topic abound. Usage can be figured out in twenty minutes of practice. There is another thing I forgot to mention. If you cannot flex the plate, you run the risk of corners digging in and leaving tracks. In that regard, a thinner plate might be easier to start with. However, not all possess the hand strength in the muscles used. Fatigue could set in. Some of those holding devices might give you some relief in the short term. That just depends on how much you plan to use the tool right away.
My two cents. My background is carpentry, so tool use comes easily. Design is where I need take pause.