1) Leave the box out and open where they can see it and move in and out of it at will.
2) Put a favorite sleeping blanket, rug, towel in the box.
3) Put favorite toys in the box.
4) Put treats, food & water in the box.
5) Leave out for a few days or weeks.
Let them get used to the box and finding "good" things in it.
6) If you play "chase" games with your cats and they run into the box. Do NOT drag them out for more play.
Let them think this is a safe place to be.
We actually use ours as a "end table", our cats play in it, sleep in it, and now simply look confused when we close the door and carry them outside in it.
...that is till they sense we are close to the Vets office....