Where To Start Design On Sweater

I want to knit a cardigan for my grandkids (toddlers) and I’ve found a lot of basic patterns but I want to add a design next to the button band and next to the buttonhole band. I want to have the button band, the pattern (most likely an 8-stitch pattern), and the reverse on the buttonhole side.

If I’m knitting a basic top-down pattern for a cardigan with raglan sleeves that starts with the collar and is knit in one piece, where would I start the 8-stitch pattern? The collar is done in garter stitch and the sweater is knit in stockinette stitch.

And would it be difficult to put the 8-stitch pattern on the sleeve? I don’t know if I’m saying that correctly, but I’ve seen patterns that have the same decorative stitch that goes down the front, going along where the sleeve meets the sweater.

That all sounds very do-able. Usually, patterns on cardigan fronts go from the lower end of the collar to the very bottom of the ribbing. It shouldn’t be difficult to run the same pattern down the sleeves, too.
You should take the pattern stitch into account when you knit a swatch to make sure you know how many sts are needed for the pattern. Cables foe example, pull in the knit fabric and so require extra sts added in order to include them.