Basically that's it, though if you want an accurate .125 slot you'll need to use a smaller endmill to cut either side of the slot,
a .125 endmill alone would make a slot probably .005+ over size due to runout and/or tool flex.
Also you'd want to take that in a number of passes at say .025-.05 deep.
You need to take some time and read some books or watch some movies on machining so that you go into this using proper techniques.
There are several books and vids linked here that could help you out.
Attention Newbies! - CNC - The Hossmachine Forum