Help changing a pattern

I have a pattern that I want to double it’s a purse pattern, with increases. How do I do it.

Cast-on 40 sts. Join into round.

Row1: Knit. Place stitch markers after the 10th and 30th st. These will be the sides of the bag. (You can mark the 20th and the last if you use a circular needle.)

Row2: Knit. Increase 1st before and after the stitch markers. (Plus 4sts, 44sts.)

Row3: Knit.

Repeat row2 and 3 until there are 80 sts on the needle.

Knit 15 rows.

Start decreasing. Decrease 1st before and after the markers. (Minus 4sts, 76sts.)

Decrease in this manner on every third row until there are 60 sts on the needle.

Cast off.

Sew together the bottom of the body.