Sew shoulder seams. Place markers 4 (4, 4½, 5) in. [10 (10, 11.5, 12.5) cm down from shoulder on Front and Back. From RS, pick up and k 26 (26, 30, 32) sts between markers. Work in garter st (k every row) for 2 in. [5 cm].
You have made the back and each front. Now sew the shoulder seams. Then lay the sweater out flat and measure down from the shoulder seam on the sweater front 4 (4,4 1/2, 5) inches and put a little pin or scrap of yarn to mark where that gets you to. Then measure the same way from the shoulder on the back piece and place a marker there. Do that on each side so you end up with 4 markers 2 on the left and 2 on the right of the sweater.
Have you ever picked up stitches before, or done what is called sometimes 'pick up and knit'? That is what you are going to do. If you have never done it before there is a video about how to do it on this site in the video section up at the top of each page. I think it is classified as an advanced technique, but look around and you'll find it.
Turn your sweater so that you are looking at the right side of the sweater, and have the outside shoulder seam side at the top and the two markers for that side at left and right. Start at the right marker and draw up a stitch onto your working needle right at that marker. You will want to pick up 13 stitches between there and the shoulder seam, then 13 more from the shoulder seam to the left marker. You are going to make the sleeve with those 26 stitches working from the shoulder to the cuff. After you follow all the instructions for making one of the sleeves do the same on the other side.
When you finish the sweater you fold everything so that it looks like a sweater with the back in the back and the fronts on top, open in the middle. The little almost rectangular sleeves will fold to make the sleeves. Then you sew the side seams from waist to underarm and then the sleeve seams probably from cuff to underarm. The sleeve seams will be under the arms.