Generally speaking, unless the hatch is thick balsa, then it should be made with the grain going in the short direction. You can put dowels in the balsa hatch and drill through them to provide hard points for screws. You can also put some squares of plywood on the underside of the hatch and counter-bore the balsa down to the ply for the same purpose.

Inside the plane, you can put squares or triangles of 1/4" plywood or hardwood to thread the screws into. Another way which I like is to glue 3/8" - 1/2" long pieces of inner NyRod into the balsa on the plane and use #2 wood screws or servo screws. You don't need to glue hardwood in if you do this because the NyRod holds a thread well.

Just drill a 1/8" hole in the balsa where you want the screw to go, put in an over-size piece of the NyRod and then put a couple drops of thin CA next to it. The CA will wick in and hold the NyRod in place. Then trim off the excess. It's neat and light.

JohnWillman was right in saying there are a lot of ways to do it. Once you've built a few models you'll learn all kinds of ways to make hatches. But, the more you know, the less hatches you need.

You should avoid hatches whenever possible because they weaken the structure while making it heavier. But they work and if you need one, you need one.

Harry Higley has a book called "Mostly Mounting." It covers some of this, but I didn't think it was worth paying for because there was nothing in it that I didn't know already or felt I had a better way of doing. But for a new builder, it will give you a jump start on some techniques.