When finishing a knitted sweater, deciding how to make the button band is usually more a matter of personal preference than anything else. Many button bands called for in patterns are knit sideways in a rib stitch, although there are patterns for button bands cast on from the bottom of the sweater edge and knitted on as you go, if you prefer to do it that way. The simplest method for a knitted on button band, however, is to knit it as you go with the sweater.
Knit a square that is several inches wide and several inches long with your yarn and needles in the pattern you are using, as well as a garter stitch border on all sides, to calculate your knitting gauge, or your stitches per inch of knitting. The precise dimensions of your swatch are not important, but you need to have several inches of fabric in order to get an estimate. Lay your ruler along your finished swatch in several places and note the number of stitches per inch you get. It should be fairly consistent. Lay it vertically along the swatch and note the number of rows per inch as well.
Cast on the required number of stitches per your swatch's gauge and the desired dimensions of your front sweater panel, plus whatever you need for your button band. For instance, if you have a sweater panel that is 10-inches wide, and you want a 2-inch button band, at a gauge of 4 stitches per inch, you need to cast on 48 stitches. Place a marker at the 2-inch mark (8 stitches in on one side, depending on whether you are knitting the left or right panel) that delineates your button band section.
Knit your sweater panel in pattern, and knit your button band in an expanded garter stitch. For expanded garter stitch, knit one row of your button band stitches, purl the next, purl the third row, and knit the last. Repeat these four rows. This will give you two rows of knit followed by two rows of purl when viewed from the right side of your knitting, like a rib pattern.
Add button holes in the purl sections of your button band pattern by casting off 2 stitches (for a small button) or 4 stitches (for a wider button) in this manner: in the second row of the button band pattern, purl 4 (2) stitches in pattern, cast off 2 (4) by passing both stitches over to your right needle, passing the first stitch over the second. Slip another stitch over to the right hand needle, again passing the first stitch over the second, until the required number of stitches are bound off, replacing the last stitch that had another passed over it back onto your left hand needle. Loop the yarn 2 (4) times over your right hand needle to replace the bound off stitches and purl the remaining 4 (2) stitches on your left hand needle. Knit the next row as directed, including the yarn overs and continue in pattern.
The position and number of button holes in your button band will depend on the number of buttons and how far you would like to button your sweater. After you calculate how frequently you would like to add a button hole, be sure to measure your knitting carefully to get them placed right.