Here's a tutorial that will show you the basic construction. This is where I learned how to make them. Ignore the Fair Isle color work. You don't need to get that fancy. You can do these in a solid color. She has a link for the actual pattern. You have to sign in with your Google Gmail signature. I believe she used a #5 needle and cast on 40 stitches. That was an extra large adult. I get by with a cast on of about 32.
If you're doing fingers in these, there's a lot of seaming. But it's well worth it. For one thing, having fingers in these keeps your hands warmer in cold winter temps rather than doing these as a fingerless glove with just the fold over mitten top. After I knit my first pair, that was it. I was hooked on these. They're so handy for fishing out parking meter change and keys. You don't lose your gloves because they stay on. They're excellent driving gloves when the temps get below zero.