I've installed an Aprilair 700 humidifier, however, in order to power my humidistat I must install a 24 V transformer. My problem is how to connect or install the transformer to an existing 110 V outlet. I would appreciate any instructiona on the installation.

First off here is a important number you should have.
You may contact a technical support representative by calling 1-800-931-0014.
Here is a copy of Aprilaires factory wiring diagram showing various ways to hook up the electric to 110 volts. Your furnace might already have a tap for the humidifier on the circuit board. If these wiring diagrams do not solve your wiring problem I suggest you seek out the advice of a professional. This 110 volts and it can hurt, kill you, or damage the equipment. Most homeowners are not qualified/experienced enough to do this wiring but since you asked for help I found the wiring diagrams for you. As you can see these units are designed to be wired directly to the furnace not a outlet. The reason for this is to control the unit so it does not run except when the furnace is running in the heat mode.

Aprilaire Humidistat Wiring Diagrams