Reducing pattern size

I want to do a baby blanket that calls for Sport Weight yarn and I want to use Worsted - Just finished one with this same problem and
it was too big! I could have stopped sooner but the way the pattern went it would have looked like it was horizontal when it was supposed to be vertical. How do I figure out the stitches that are the actual
pattern (between the borders) so I can reduce the number? I’m self
taught, so I’m sure a lot of holes in my abilities.

In general, sport weight is about half the thickness of worsted, so roughly half to 60% of the sts. But it needs to be a specific number because of the stitch pattern. So if you can link to the pattern that would help us be more specific as to how many sts to use.