If you keep your Tillandsia indoors, it will be healthiest by spraying them with water 2-3 times a week. Plants that are constantly being dried out from A/C or heaters will require more moisture. A simple mist from a spray bottle will do the trick.
If you are unable to give them the spray 2-3 times a weeks, you can also soak your plants. This can be done by placing them upside down in a cup of water for approx. 1/2 to 1 hour. Try to avoid soaking their feet (roots or base). Take care to gently shake any excess water off the base of the plants, as sitting water can cause rot and damage/kill the plant. Never let your Tillandsia sit in water for prolonged periods of time or let their base sit on a wet surface for long period (eg in wet sand or soil). This is one of the most common ways people kill their Tillandsia - too much water sitting inside their plant or sitting in wet ground - death by root rot.
You will notice if your plant has received to much water when the base of your plant is feeling soggy and moist. The leaves will change to a darker brown or mossy green colour. If this happens, try and dry the base of the plant out immediately in the sunlight. Alternatively you can softly pull these leaves out to avoid them getting the remainder of the plant to wet.
Remember that each plant variety is different, and will require different watering schedules than others. Never put your Air Plants in soil, it will harm them!
My favourite way to water is to put the plants outside when it rains. Just pop them on a tray and leave them in the rain for a good soak, then let them drain properly and put them back where you had them!