Velcro rollers are ideal for adding volume to flat, straight hair. The key to getting them to work lies in choosing the right size rollers. For tons of volume, choose the largest Velcro rollers that will stay in your hair.  This obviously depends on the length of your hair. If you have shorter hair, you will need to use smaller rollers, whereas if you have longer hair, you can use bigger rollers. Here's how to go about rolling your hair:
1. Start by sectioning off your hair into sections that are about as wide as a standard roller. One section should be directly on the top of your head, starting at each temple and going back to the crown of your head. The next section should start at the top of one ear and continue all the way around the back of your head to the other ear. Finally, the last section should include the remaining hair on the back of your head. For each section, twist the hair and clip it out of the way until you are ready to work on that particular section.
2. Starting with the bottom back section, begin rolling the hair. Take a piece from one side of the section, spray it with hairspray and roll it around a large Velcro roller, making sure to position the roller vertically and to roll it toward the center of your head (away from your face). You should slide the roller out to the very ends of the hair before you start rolling rather than starting in the middle and wrapping the hair around the roller. This will result in a smooth, even curl. The roller should stay in place on its own next to your scalp without the need to secure it. However, if it feels loose, you may want to use a small bobby pin to hold it in place. Continue rolling the back section, making sure to roll all of the rollers in the same direction.
3. Unclip the next section and roll it in the same way, starting at one side of your head and rolling backwards away from your face until you reach the other side of your head. Roll this section in the opposite direction from the last section. For instance, if you started rolling the back section from the right side of your head toward the left, roll the ear-to-ear section from the left side of your head toward the right.
4. Finish by rolling the crown section of your hair. Starting at the back of the section, roll the hair toward the back of your head. As you approach the front/bang area of your hair, slightly angle the rollers to one side or the other rather than rolling them straight back.
5. Spray the entire head with hairspray after you are done rolling. Then, apply heat to the hair with a hairdryer, making sure each roller is adequately heated.
6. Allow the hair to cool completely before removing the rollers.  Unroll each roller individually. Once all of the rollers are out, use your fingers to style your hair. Spray the roots with hairspray to help them keep their volume all day.
Check out this video for step-by-step instructions: