Your hair length depends on the size of the roundbrush-the longer the hair, the bigger of a round brush you need. Use a ceramic brush, as that helps hold curl, body & volume better. You will also want to use a nozzle on the end of your blowdryer, as this helps control the direction of the heat onto the brush directly instead of all over.
1.) Wash and blowdry hair without round brush until it's about 80% dry. Trying to round brush soaking wet hair doesn't turn out well and will take you way too long.
2.) Apply a volumizing mousse and/or smoothing cream to your hair and start off with one inch sections starting form the bottom. (You can clip the rest of the hair up that you aren't round brushing, so that it doesn't get caught in the brush by accident.)
3.) Place the round brush at the roots underneath the hair, and as you blowdry (med heat) bring the brush *slowly* down to the ends. When you get to the ends, twist the brush around a few times so that it can curl the ends. Try to blowdry vertically, as that will create more volume. Another way to achieve more volume is to overdirect that hair. For instance if you are blowdrying the top of your hair, direct the brush so it's past a 90 degree angle going towards the front of your head.
3.)When all is finished, if you'd like extra volume, tease the underneath of the top layer of your hair and smooth over. Finish with a light hairspray. For less volume, don't tease, and don't overdirect the brush as much.
Good luck!