1.
Knit the first row by alternating the number of knit and purl stitches you need. This means for 2x2 rib you would knit two stitches and then purl two stitches, repeating this until you reach the end of the row.
2.
The stitch pattern established in the first row will need to continue in columns to create ribbing, so as you work across the other rows you'll need to knit every knit stitch and purl every purl stitch.
Tip: If a stitch appears to have a horizontal strand of yarn across the bottom of it, this will be a purl stitch and if it doesn't have this horizontal strand it will be a knit stitch.