For further explanation: http://www.123teachme.com/learn_spanish/counting_from_100_to_1000/#!/1
Practice:
1. https://quizlet.com/297906/scatter
2. http://study.com/academy/lesson/spanish-numbers-101-1-000-practice-activity.html
3. For this website, the top half is an explanation of how to count but you can scroll down to the bottom of the page for practice: http://www.rocketlanguages.com/spanish/learn/numbers-in-spanish/