Baby raglan sweater-button hole band

I am knitting a baby raglan sweater and all the pieces are put together. I am now left with 8 st which were put on a holder for the button band on either side. I have never done this and the shower is tomorrow so I am hoping someone can help.
-The directions tell me to cast on then rib 9. I am not sure why I am casting on a new stitch? Is there any reason for this or should I just rib the 8 I have?
-I am doing it in rib (k1,p1)? Not garter st? it seems it would be better in garter…but what do I know.
-oh, on another side it said to make sure you slip the first knit stitch for a cleaner edge? Is this recomended? are we talking the outside edge?
Thanks ahead of time!

You can do it in garter if you like, but it’s shorter than stockinette, so you would have to do more rows to get it to match the length of the sweater front.

You could leave just the 8, but ribbing draws in sideways so one more st would make just a little more width; also there may be a reason to have an uneven number of sts when you do the buttonholes, read ahead and see how they’re done.

Many patterns and knitters say to slip the first st (outside edge) for a neater edge, but mine are looser and messier than if I just knit or purl the edge st and pull the yarn a bit after I do it.