How complex is your part? I recently did some manual g-coding just to see how hard it was. Hard enough, but not impossible. I used my 2-D CAD program as an aid.
First, I drew a symbol, a circle with center crosshairs, the diameter of the endmill I would use. I copied and placed these symbols at strategic places on my CAD drawing. I then "connected the dots", the centers of the symbols, to define my toolpath. Placing the cursor on the center of the symbol I wrote down the coordinates, in the order that I wanted the cut to be made.
I then typed the coordinates and added the g-code commands into a text file that was sent to the CNC system.
This is fresh in my mind because the process will appear, along with illustrations and photos, as well as my experiences with a CNC program in my column, The Mechatronist, in the Summer 2011 issue of Digital Machinist magazine.
Weston Bye - Practitioner of the Electromechanical Arts - Author of The Mechatronist Column, Digital Machinist magazine