My mom cooks by feel. She might – might, mind you – look at a recipe as she's making a meal, but the recipe is just kind of a starting point for her. She'll omit or add ingredients until what's in the pot is her own, rather than an exact replica of the recipe she started with.
Team Challenges should be handled similarly. Sure, there are instructions. Detailed ones, even. But don't get hung up on replicating each exactly! Grant extra time for beginners, adjust materials to suit what you have on hand, add a penalty if you want. Ms. Frizzle is doing just that as she presents challenges to her robotics group, and as she tries out the challenges, she's even thinking of ways to alter them to make them different or more challenging. Bravo!
What if you're trying to put together materials for your group and you realize that you don't have straws? Improvise! As a matter of fact, in Chapter Three I even list possible substitutes for the materials commonly used in the challenges. These challenges are intended to promote creativity – don't be afraid to use some in presenting them to your group.