Currently attempting the Deramores All-In-One and have come to a halt while attempting the sleeves.

pattern reads:

cast on 29 sts.
work 6 rows in 1x1 rib as given for the Legs
and at the same time inc 1st in final row. 30 sts

I have completed the rib and have gotten confused at the next part.

Commence stripe pattern starting with a 15th pattern row at the same time inc 1 at each end of every foll 6th row to 40 sts.
Cont in stripe pattern without shaping until 42 rows of stripe pattern have been worked, ending with a 8th pattern row to match body at underarm point.

I have worked out that after working 42 rows of stripe pattern I actually end up with 37sts instead of 40sts.
maybe I’m just not reading the pattern correctly but I’m totally lost.

Someone please help.

Here is the actual pattern. I’m working the green sts.

Remember that you’re increasing at each end of the row. That’s 2 increases per increase row. You’ll increase on rows 6,12,18,24,30 and that should get you to 40sts. Work the remaining rows without increases.

Ah, thank you so much!

Can someone explain to me what the "making up’ part is asking me to do please.

This video shows seaming with a backstitch. You can use it along the side of your knitting or across the top. The instructions you posted want you to seam from a marker on the center seam to the waist.

Whether you press the pieces or not is up to you but do see the instructions for your yarn. I’d probably just wash this when finished.