How to Knit into Front and Back
Knitting into the front and back of a stitch is one of the easiest and most common knit increases.
On a knit row, work into the front and back of the next stitch: knit into the stitch and before slipping it off the left needle, twist the right needle behind the left and knit the same stitch again through the back loop. Slide the original stitch off the left needle -- there are now 2 stitches on the right needle made from the original one.
On a purl row, the method of increasing is similar. Purl into the front of the next stitch, then purl into the back of it before sliding it off the left needle.
Below: LW3949 Teen Endless Circle Vest Knitting Pattern