They should be drawn with a 4H pencil while the lettering should be darker, drawn with a 2H pencil. And almost anyone can achieve it, regardless of whether those are qualities they personally possess.
Guidelines are very light and almost invisible. Youngsters who are dab hands on the keyboard may have little impulse—or opportunity—to practice their printing. All it takes is determination, time, and—as taught by Doug Patt in his How to Architect series, above—more tools than can be simultaneously operated with two hands: a parallel rule or t-square a small plastic triangle customized with bits of tape a.
Let's remember that letters are really just shapes. Historically, the most legible American penmanship belongs to architects. Or is it doomed to go the way of cursive? This style of writing was famously used by architectural illustrator Frank Ching, whose books on drawing and graphics—including Architectural Graphics—are standard issue material for architectural students.
Multiple people would be making their marks on any one blueprint—writing new notes, correcting existing notes, sometimes even just changing a word or two in the middle of a sentence.
Practice is necessary to develop the skills needed to letter legibly.