Being able to work an Aran sweater as only your 2nd project proves you're no dummy! Good for you.
-Work each side of neck separately
In order to have a proper fit and to sew the shoulders together, you need the actually neck opening a bit lower than the top of the front & back pieces.
-K3 and turn, leaving rem sts on a holder
You're working on the right shoulder at this point, so knit the first 3 sts and just stop (but leave on needle with yarn attached). Slip the remaining 24 sts onto a stitch holder. (If you don't have one, just run some different coloured yarn through them while removing the other needle & tie it off as a loop, so you don't lose them & your work doesn't unravel. It will be easy to slide a needle back through them later.)
-Dec 1 st at beg of next row
Go back to those 3 sts you worked, and turn your knitting so you're working back away from the saved sts on the wrong side. Decrease 1 right away & work 1, so you have 2 sts left. (Doesn't say what stitch to use, purl is probably okay since it's just 3sts.)
NEXT ROW (RS): K2tog and fasten off.
Straight forward, I think.
-W/ RS facing, rejoin yarn to rem sts, cast off center 21 sts, K to end.
-Complete to match first side, reversing shapings.
Slide the 24 saved sts back onto the needle you're working with. You will now work across to the opposite shoulder, so cast off the first 21 sts for the neck, and K the last 3 sts. Turn your work to the wrong side. Using the same stitch you used at other shoulder, work 1 and then decrease, so you end up with 2 sts again, but as a mirror image of the other shoulder. (This shaping would be more obvious if you were working with more sts and a few more rows.)
NEXT ROW (RS): K2tog and fasten off.
There, you did it! Hope I explained that logically.