Simple. You use the quill to draw/write. Press the bottle with the color you want. Adjust the pen size to the one you want. Unfortunately, I haven't gotten over sixty, so it will tell you that you cannot use it. Press the clear button if you want to delete it. Press the eraser button to specifically erase a part of the drawing/writing. The eraser size is the pen size. Happy drawing/writing!
Credit to Scratch existing in the first place so I could make this. Also credit to the help section so I could figure out how to work the variable thing.