Well, if you have already crocheted around the sweater and you are happy with the result, then you are doing just great! I admire your sporting spirit!
I think what that pattern is telling you to do is to work 2 chain stitches, then skip two stitches before working your next double or single. So you would be crocheting along in each stitch, then at the point where this instructions indicates, instead of working single or double crochets into each of the next two stitches, you would skip them and work two chain stitches to span that space, then continue crocheting in the third stitch.
I'm guessing you would do another row or two after the buttonhole row. For that row, you would work into the two chain stitches just as if they were singles or doubles.
You are basically making a hole by skipping some stitches and using the chains to span the space.
I hope this is helpful and not too confusing.