How do I run a 100 amp service off a 200 amp service? The part I need to know is where to hook up the aux. service in the 200 amp panel.

You put a 100 A breaker n the main pane. Run 4 wires to the sub-panel.

If the sub is in a detached structure:
1) you need a disconnect. You can use a 100 A or even a 200 A panel and use the main as a disconnect.
2) You need a ground rod.

The sub panel most likely needs a ground bar kit and the neutral isolated from ground (usually remove a screw).

Neutrals and grounds are separate in the sub-panel.

How do I run a 100 amp service off a 200 amp service? The part I need to know is where to hook up the aux. service in the 200 amp panel.
You don't run a service off a service. You run a feeder off a service.
When you say where to hook it up, what do you mean? Where on the breaker?

This question is extremely vague. Can you give more detail about what it is you are doing?

I have a cuttler hammer 200 amp serv. To the house. The breaker box is mounted outside on the brick wall next to the meter. I want to run my wires from that to a 100 amp box in the garage. I have the right wire to do this according to a friend who is a master elect. I moved from where he is thoufg. ( was going to do the same thing in prev. home. Where and how do I hook up the wires in the 200 amp box? I can figure the wiring in the 100 amp box.

The two hots go to the breaker, both the neutral and ground go to the neutral/ground bar.

Jbiggi,

I do not mean to be offensive but you really need to spend some more time telling us what you are trying to do.

For example, how far away from the main panel will the subpanel be. Will it be in a detached building or in a area within the same structure.

How do you plan to lay out the wiring from the sub panel.

You say you have the correct wire, can you please tell us what wire you have and how do you plan to get it over to the sub panel?