You're doing it in your lap...and the head or file is not stable. Any pressure from your file hand causes the head to move away. It's curling all right.
Go to a stair railing, or a fence, or a table.
Hold the head on the edge of the table..run the file on it so the head won't move. It's as simple as that. Once the head is stable you'll see that edge come to a little foil pretty quick.
Strop on your jeans, or a piece of cardboard gently, until that foil falls off...back and forth till it just breaks loose. STOP!