Instructions for making a square collar rather than round

I am making my partner a sweater coat with golf things on it (golf balls along bottom, golf bags on left and right side, and pic on back). I am working with a very old pattern and even the newer ones like this: have the rounded collar. He would like a square collar to look more like today’s collars. Any help would be greatly appreciated.
Instructions for rounded collar as as follows:
With main colour cast on 7 sts.
1st row: K1, inc 1 st in next st, K3, Inc 1 St in Nest st., K1
2nd row: knit
3rd row: K1, inc 1 st in next st. Knit to last 2 sts, Inc 1 st in next st. K1
4th row: Kint. Repeat 3rd and 4th rows twice (15 sts on needle)
9th row: (outside edge) K1. inc 1 st in next st. Knit to end of row
10th row: Kint. Repeat 9th and 10th rows to 23 sts on needle.
Knit even in Garger st until work measures 13 1/2 inches.

Do you mean more of a pointed collar, like this one?

You could pick up sts around the collar and knit onto the sweater that way. You won’t need much shaping this way.

Thank you.
I might try that, but would prefer to be able to knit a square collar and then it would fit in with the pattern better, or maybe this is just my inexperience talking. lol

You can do that. You could modify the directions for the collar you have. Cast on the full number of sts and knit without increases so that you have a rectangle to sew onto the neckband.

Thank you!! I am going to try that!