How do I draw the line I know is right?!

Drawing the line you want is about two things: Increasing Dexterity (Mechanical skills/hand-eye coordination/getting used to the pen) as well as a build up of commited knowledge of the form enough to visualize you want to draw. These are separate-ish abilities.

Drawing Skill requires = DEX + INT!

INT: You need enough of a mental image and understanding of what the form looks like before you draw. You can't draw what you don't know. I go over this thoroughly in Chapter 1 (Visualizing in 3D) and Chapter 3 (Research)
DEX: Increasing DEX means your brain is able tell your muscles to tell your pen to do exactly the line you want. I don't really go over this much in Class 101 besides getting used to your grip, but I think it is worth mentioning and going in depth with how this is ALSO a skill you can practice! Remember when we were kids and we struggled to get all our letters to look not like crappy and wobbly? Or like, remember when we were younger and cutting with scissors was weird, or using chopsticks was like, a struggle? You might've been a fast learner and completely forgot what it was like NOT to know how to do these things, but like, think about how an athlete doesn't make any unnecessary movements and is really familiar with how to handle the ball, etc. Drawing is no different. Your mechanical skills and dexterity increased on all those tools, and it will increase the more we think about practicing precision.

Increasing DEX: Patience

Be mindful and patient with yourself. Sometimes you lose dexterity just because you're stressed. We tend to be clumsy and make more mistakes when we rush or when we are tired, or when we are putting too much pressure on ourselves. Make sure you're clear of the noise as much as possible!

Tips for patience:

Breathe
Don't take it out on the pen— grip the appropriate amount and no more
Relax your shoulders and try to draw from the shoulder and elbows more than your wrist

Increasing DEX: Precision

If you're having trouble drawing the line you want, here are some extra practices:

Point-to-point exercise:

Step 1: Draw two dots, one on each of the side of the paper
Step 2: Draw the long, long straight line all the way across the page connecting one dot to the other. Try not to be wobbly, or broken, or overshoot
Step 3: Fill the page! The more you do the more DEX points you get! >:D