Okay. You will probably need to pick up some stitches and then join to work them in the round. There is a video about picking up stitches in the "view videos" drop down menu in the "tips" section. Scroll down till you come to it. I would pick them up on 3 double pointed needles or two circulars or use magic loop. All of those techniques are shown in the "advanced techniques" section of the videos.
Sometimes the pattern will tell you exactly how many stitches to pick up. I generally ignore that and pick up the number that "want" to be picked up. Start picking up the stitches at one of the shoulders or in the back. I usually use a needle two sizes smaller than the needle I knitted the body of the sweater on. On areas where the area to be picked up slant (\ or /, like the sides of the neck edges), or are vertical, I try to pick up one stitch in each stitch for three stitches and then skip one. 1, 2, 3, skip (It is good to pick up the same number on each section that is alike on each side. By that I mean like along the left neck in the slanting section to the shoulder, if you get 14, pick up 14 on the right neck to the shoulder too.) The video will help you with where to pick up stitches (I hope). On the flat (horizontal)areas like right in the front or back neck pick up one in the middle of each stitch. Those are the general guidelines, but you have to consider the type of ribbing you want to do. If it is 1X1 ribbing you need to end up with a multiple of 2. If it is 2X2 you need a multiple of 4. So you can fudge a little to make it turn out the number you need.
Keep in mind that you pick up the stitches from the outside and what you see is what you get. If you pick them up and then work a row or two and you can't live with the way it looks take it out and try again. You may be able to tell right after you pick up the stitch whether or not its placement looks good, but maybe not.
Here is a free online knitting book
. You want chapter 24.
I notice she says to use the same
size needle as the body of the sweater, or did for the front bands (I didn't look real closely). That is different than the lady who taught the finishing class I went to. It depends on personal taste I suppose.