Summarize:

Whitefly traps are available commercially. They are usually bright yellow strips of sticky cardboard, staked or hung above plants. As you might have guessed from this description, they are easy to make yourself as described below. Note that these traps will only catch adult whiteflies, and young wingless nymphs will still remain on the plants and damage leaves. Combine this with other treatments for best results. If you'd rather make your own traps, start by cutting  plywood or masonite board into strips roughly 12 inches (30 centimeters) long and 6 in. (15 cm) wide. This size should be suitable for most plants, but you may use smaller ones if you only have a small flowerpot or two to protect. Use any type of paint to make your traps bright yellow, on both sides. Adult whiteflies are attracted to this color. Petroleum jelly mixed with an equal amount of mineral oil or detergent creates a long-lasting sticky adhesive. Alternatively, use a heavy-grade motor oil or a commercial product such as Tanglefoot. Whichever substance you choose, be sure to coat both sides of the trap generously with a paintbrush, so any whitefly landing on the trap will be stuck. Hang them above your plants, or stake them in the ground near your plants. The stakes should be at the same level as the leaves you see the whiteflies feeding on. One trap for every two large vegetable plants or flowerpots is a good guideline. Hopefully, there'll be plenty of dead whiteflies on the trap, along with dead leaves or other debris that gets stuck to them. Wash with soap and water or scrape the traps regularly to remove these, and reapply the sticky substance so you can catch more pests. After the whitefly population is mostly dead, and you only see a few on the traps, take down the traps. Because the traps also have the potential to kill whitefly predators, they are not recommended for a low level infestation, unless the predators fail to keep the whiteflies in control.
Purchase whitefly traps (optional). Cut your own traps instead. Paint your traps bright yellow. Coat both sides of the trap with a sticky substance. Place them above plants. Clean the traps periodically. Remove the traps when the whiteflies are mostly gone.