How do l cut a sweater to make it a smaller size?

This link should help you, Elsie:

https://www.ehow.com/how_10031897_make-big-sweater-smaller.html

I lost 40 pounds and altered some of my sweaters. I turned it inside-out and ran a seam along the existing seam on each side. A few of them also needed shoulder seam alteration.

Elsie: I must admit -- I kind of do it the lazy way --- wash in warm/hot water and then put in dryer! (Mostly tho... I do this by mistake with other garments! LOL)

The kind of tough part will be dealing with the sleeves. Are the sleeves also a little too big? Because if so, you could clip the sweater inward, and run a line of stitches up from the bottom on each side and then down through the sleeve. You could also add some ruching, either at the back or along the sides, kind of like this: http://www.alazygirl.com/2014/04/no-sew-turn-loose-shirt-into-rushed.html#_a5y_p=1561450

Put in darts or take it in on the sides.

You could try sewing in the side seams a couple of inches. I would use a baste stitch first to see how it looks and then follow through with a more permanent stitch if you are happy with the results.

You could take the sides in or get some clips and clip the back a little. Or try shrinking it in the dryer

What is the fabric content of the sweater? If it is natural fibres such as wool or cotton, it should shrink when dried with hot heat, such as in a dryer.

Where exactly is the sweater too large - neckline, in the shoulder width, sleeve length, or body?

Posting a picture of the sweater would help us to see what you are starting with.