Use a fret distance calculator with a scale length you want and the number of frets you want. This will give you a length of your neck, nut to heel.
Then draw a center line on a big piece of posterboard or something. At the top of the line draw a line that is your string spacing at the nut.
At the bottom (if your scale length is 27, then at 27" from the nut line, draw a line that is the string spacing of your bridge. You can find this out by deciding which bridge you will buy and look at the string spacing spec.
These lines should be centered. Once drawn, Draw the outside lines connecting each end to make a big trapezoidd. Basically you have plotted your bridge line, nut line and have drawn in the 1st and 6th string.
Now the neck length. Using wfret, i'm showing a 22 fret neck would be 20.14" from nut to heel for a 28" scale.. 19.42" for a 27" scale. A 24 fret neck would be 21" for 28" scale or 20.25 for a 27" scale.
On your center line, pick which length up there you want and measure down from the nut line that far. That is the end of your neck and if you draw a horizontal line and make it exceed the outside lines by 1/8inch on each side you have a calculation for your neck width at heel. In fact, at this point just do the same at the nut and you will have your entire neck drawing dimentioned and if you were careful enough you can use it to build your template off of.
Hope that makes sense. It's not that hard if you just draw it out. The lengths i came up with were derived from WFret.. a fret distance calculator. You can find it online.