You need to cast on stitches at the beginning of a row. You can use the knit cast on or a cable cast on (shown under the Free Videos tab at the top of the page, Cast Ons). Cast on 4sts, knit across those 4 and the rest of the row, turn and cast on 4sts and knit across those 4sts and continue across the rest of the row…Here’s another video that may help too.
The same goes for the cast off. Cast off as you normally would by knitting two sts and lifting the first st over the second on the right needle. Continue until you’ve lifted over 4sts and then knit to the end of the row, turn and cast off 4 sts at the beginning of the row. The cast ons and cast offs are offset by a row, but it won’t be noticeable at all.