Whenever you are knitting and want to widen the shape of your fabric, you need to increase the number of stitches in each row or round. There are a variety of methods to do this, but one I frequently use is the lifted increase.

Lifted increases are made by working an extra stitch into the stitch that's two rows/rounds below the row/round you are knitting. Adding a stitch this way doesn't create a noticeable hole or bump in your fabric.