The pattern calls for casting on in the middle of the work. So there are already 40 or stitches on the needle and the pattern for the row says "Cast on 12, knit to the end of the row".
I think I know how to do this, I just want to be sure. So my 40 stitches are on the left needle. I knit the first stitch, and slip it back from the right needle to the left needle and now knit that one. Do that 12 times.
Then just knit the row 40+12 stitches.
Is that it?