To 'work' sts, you knit or purl them according to what you should do in pattern. Decreases are specifically mentioned. So 'work 4 sts' means to k4 or p4. To do garter st in the round you knit on one round and purl on the next and alternate them.
It's not allowed to post an entire pattern, but copy and pasting just a few rows so we can help you is fine. On the part you put in, after you've done the 8 garter sts in the center front, on the next round you're going to split the front and will work flat. So work to the front, turn to the ws and k 4 sts, purl the rest to the last 4 and knit them. Turn to the RS and k4, dec, knit around to the last 6 sts, dec and k4.
Keep repeating these 2 rows - purl a WS row except k the first and last 4 sts, knit the RS row with decs inside the first and last 4 sts. Do them until you have 12-12-13-13-14-14 decs, then switch to doing the decs every 4th row instead of every other one and do it that way 6 or 7 times.