Need help on my third knitting project

Hi everyone, just need an explanation to this pattern for a little girl’s shrug,…I’m stuck on trying to understand this line of instruction. It states to;
Work 1 row. Cast on 12 sts at side of edge on next row and dec0[0:1] st at front edge.
As I interpret it I work a normal row of stitching and add 12 more onto the next row but where exactly do I do the decreasing for the front edge will it be with the same row I just increased or do I do another row for the decreasing?
Thanks, will be a great help if anyone can lend me an explanation for this :knitting:

The dec would be at the other end/edge of the row that you CO at the beg. That is only if you’re making the largest size, you don’t dec if you’re making either of the other sizes.