Options:
+You can glue a ribbon or a cut edge into the box so your cat can get in and out easily. Make sure the entrance way is not big enough that your cat can slip through.;
+You want it big enough for your cat to fit in perfectly, but still small enough so that your cat can't get out. Build the entry way in the box just enough so that the cat cannot jump out.;
+You want the entrance way to be tall and narrow. Use a tape measure/ruler to measure where the opening is so you have an easy to access way out of the box.;
+This must be large enough to enable your cat to fit through. Make sure it's not too small.;Choose from options above and answer: What most naturally follows?

How to make a comfortable bed for your cat
Start by getting a cardboard box your cat can fit in perfectly.
You don't want a box that's too small. That's just uncomfortable! Make sure the box is much bigger than your cat, so he/she can move around a little, and it'll be comfortable!
Cut an entrance way in it.
Answer:
This must be large enough to enable your cat to fit through. Make sure it's not too small.