A humidity tray is pretty simple.
1. First get a flat rectangular backing pan like this:
2. Then you need some small gravel or marbles or decorative glass balls - something that is not water absorbent. The bottom of your orchid pot will sit on top of the gravel and the water level will be below the top of the gravel.
3. Make sure to keep the gravel or marbles (etc...) clean and the water level up. A fan blowing on your orchids and an air conditioner will reduce the water level pretty quickly.
4. Also, it is a good idea to group many orchids together on a humidity tray. The close proximity to each other will increase the humidity around the plants.