Answer:
To achieve this, he should use thin, light construction lines called guidelines which help to regulate the letter height.
So the word that should go in the blank is, guidelines.
Step-by-step explanation:
On Edmentum, this is what the lesson says:
"Guidelines: Guidelines are thin, light construction lines that help to regulate letter height. These lines are drawn lightly so that they will not reproduce on a blueprint copy, and they never appear on a final copy or blueprint. Vertical guidelines help to keep letters from slanting, while horizontal guidelines help to maintain the height of the letters."