Hi and welcome! When you see AT THE SAME TIME, the directions that follow are to be carried out simultaneously and the caps in the directions are to alert you to that so that you don't do the steps one after the other. In your pattern, the "at the same time" directions could be done a couple of ways but maybe the easiest is this way:
On a RS row, knit across the row, decreasing 3 sts across (for the smaller sizes). So space the decreases approximately evenly over the sts on your needle and don't put the decreases in either edge 3 garter sts.
On the next row, a WS row, knit across the row, placing markers as the directions give. I find it easier to subtract the stitch number and find out how many sts from one marker to the other rather than counting from the beginning as in the directions. So, again for the smallest size, the markers would be placed on the needle after 4sts, then 9sts, 8sts,9sts leaving 4sts at the end.
A poncho with a hood sounds like a good project. Have fun finishing up!