How do I raise the humidity in my grow tent?

Slowing the fan is the cheapest first step, since it holds more moisture in. Plants themselves add humidity as they grow. For seedlings, a humidity dome helps. If you use an ultrasonic humidifier with hard tap water, expect white dust on the leaves, so use filtered water.

See humidifiers, for example the Clivex Eco Cloud 4 L or Dutch Masters 260 ml/h.