Horizontally knit bands are usually made by picking up stitches along the center front edge, creating a horizontal band by knitting at a right angle to the sweater body for an inch or so. If your pattern gives you a total number of stitches to pick up, you have to ration the total out in equal sections along the front edge and pick up stitches evenly along the edge.
Divide the front edge into equal sections about 2 inches long.
You can measure out sections with a ruler, but it’s better to count rows. Use safety pins to mark them.
Count the number of sections you’ve marked.
Divide the number of stitches the pattern says to pick up by the number of sections.
Pick up that number of stitches between pins.
For example, if the pattern tells you to pick up 120 stitches and you’ve made 12 sections, pick up 10 stitches in each section.
You may want to test your band by picking up stitches in a few sections (about 6 inches or so total) and knitting a band from them to ensure that the band doesn’t distort the edge.