The best bet is, start with some large graph paper that has the 1/4 inch squares.
Make a very distinctive centerline, measure to scale the length from the nut to the intonation point, pickups, etc.
After, not before, draw the perimeter of the body style that you want. If it's an Explorer you want, then trace an explorer. If it's another body style that is already in existence, then trace it. If you have a body style of your own design, draw it on there taking into consideration the maximum outer dimensions of the body blanks listed on Warmoth's site. From there, you can make known your intended location for vol, tone, p/u selector switches, etc... Just try to keep things in a realistic perspective, taking stability issues and balance issues into consideration.
That's the best way to start out.
P.S. Just for the record, I want a mahogony body blank/quilted maple top for a 7string 25 scale with the neck pocket, 2 hums routed out, as well as the recessed floyd cavity, & I'll cut out the body, & route for the controls. Additionally, I'd like a 7 string 25 scale padouk neck with a ziricote fingerboard with dunlop 6100 SS frets, with a wide paddle head so I can cut out my own 4x3 headstock. :headbang1: