Note, when you first start a round rug, you may need to increase often and there may be visible large holes or gaps that make it easy to see where an increase is needed. As you go, the holes may not appear quite so large, so don't use the actual hole or gap as a guide. A better guide is the distance between the knots. When the next loop begins to appear too far away, increase.
The easiest way to increase is to add an extra knot in one space, as shown in this video. I have another technique to add that extra knot which gives a neater appearance and doesn't leave the hole looking so big, but it can be tricky. I'll post that advanced technique if there is interest. (Update: For the other technique click HERE)
I hope this video helps. If you would like more information on increasing, or if your rug is curling up, this blog post may be helpful: 3 Reasons Toothbrush Rugs Curl Up