Help please!

I'm a relatively new knitter who has figured out that I have been able to rise to new challenges by trying harder projects. And.... I'm stumped. I'm making my first sweater ever, and it's the EZ February Baby sweater on 2 needles. I understand the pattern, got the increases just fine, but I've never done sleeves.

This is the part that confuses me:
At 41/2 inches, work 25 sts then work 25 sts back and forth on the next 28 sts (plus 7 sts cast on at each end of them, 42 sts in all) for about 4" for sleeve, ending with 1" of garter stitch.

What does that mean? Also, I've seen here that it is an helpful trick to add an extra stitch at the end for seaming. So, cast on 8? On both sides?

Any help would be much appreciated! Thanks!