Perfect practice makes perfect.
When you’re just practicing with no resistance, go as slow as you need to, so that you can do everything exactly as the instructor showed it.
I see this a lot: a beginner learns a move and is practicing it way too fast and skipping a lot of steps and details that were shown. Sometimes they even look pretty good because they’re going so fast you can’t really see how much they’re messing up. When you go slow, you’re probably going to look worse to the people around you, because they’ll be able to see the steps and details that you’re missing. That’s good; that’s what you want. Hopefully someone will correct you or you’ll correct yourself. A hundred bad reps is worse than 5 good reps. Arguably it’s worse than no reps at all.
Obviously it’s important to practice a technique smoothly and quickly as well, but only once you’re doing it correctly.
Related articles:
http://breakingmuscle.com/brazilian-jiu-jitsu/feel-me-flow-deconstructing-let-s-just-go-light
http://www.expertboxing.com/boxing-training/boxing-sparring/the-secret-fight-training-method-slow-sparring
http://www.bulletproofmusician.com/is-slow-practice-really-necessary/
http://www.golf.com/instruction/speed-your-improvement-try-slowing-down