For this example I am using 3mm needles.
Cast on 12 stitches and knit 4 rows of knit one purl one rib, remembering to slip the first stitch of every row as if to knit, without knitting, to maintain a nice neat edge. If you do not do this you may get a sloppy loopy edge which you won't like!
Step 1: Creat the first buttonhole
Row five, continue in knit one purl one rib for 5 stitches.
Cast off 3 stitches (in rib, i.e. purlwise, knitwise and purlwise) and continue knitting to end of row in rib, starting with a purl stitch.
Next row knit 5 stiches in knit one purl one rib, starting with a knitted stitch. Cast on 3 stitches onto the right handle needle (you may have to turn the work around, depending on how you personally cast stitches on). Try not to make these stitches too "loose and loopy"!
When you have cast on the 3 stitches on your right hand needle continue to work across the piece in rib across the left hand needle, starting with a knitted stitch.