First, when you are dragging the log over to the square does it
stay there? If it doesn't stay there then you have the log facing
the wrong way. Make sure the pointed part is facing down. (The
way it is facing can be very tricky later on with more complex
projects.)
Second, if the log does stay in place you will then see two cross
hairs and 2 squares going back and forth along the line. You
want to tap the squares when they meet in the middle. If you
time it correctly you will have moved on to the next one.