writing in Preview?

How do we use the writing tool in Preview?

I open the annotation bar and select the signature tool:

I place the cursor on the button that reads, "Click here to begin." The button goes away. I put my finger on the trackpad and try to write something. Preview places my starting point in the middle of the screen, right where the "Click here to begin" was. I try to move the cursor to the left to start writing. Preview draws a line from the starting point in the middle of the window to the left margin where I want to start writing. At that point, though, I have a squiggly line across the left half of the window that is not part of what I want to write. I pick up my finger off the trackpad and place it back down. Preview draws another line from the left quarter of the window to the left margin. Again, this new line is not part of what I want to write. I repeat the process until I get utter gibberish:

How to I use signature tool with a trackpad?

(Ignore the blank rectangle in the middle of the image. That is a later and unrelated insertion.)