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.
Pattern: Birch Grove Socks - https://shop.thechillydog.com/products/birch-grove-socks-knitting-pattern-pdf
Yarn: Schoppel Boots
Needles: skacel by addi FlexiFlips