I assume you are talkin bout doing the 18 V mod for EMGs? you can ask EMG for a diagram.....or if you do a search on 18 V and EMG on Google, there will be diagrams as well
You got to check whether the preamp/pickup can handle 18V. Most do (EMGs can handle up to 27V) It won't improve your sound, but it'll give you more headroom so the signal won't clip (ugly non-musical solid state distortion haha) especially if your pickups are damn hot or very strong playing style etc. Recommended more for basses...
On how to do it if its EMGs, juz disconnect the black wire from your battery clip -> output jack (the battery ground not the guitar ground) and join it to another battery clip which you can buy at sim lim tower or something for like $0.50. Join the old removed black wire -> new battery clip's red wire; then connect new battery clip black wire -> output jack where you took off the old black wire from. Just think of it like analogous to putting in an extra effect pedal in your effect chain... Only this is putting an extra battery in your power supply. You got to have soldering skills though, its not difficult.
I think this kind of work not worth sending to shops to do coz they usually overcharge (we repairmen need to cover costs too). It'll probably cost you anything like $5 to $20 ( 8O ) if you send it in depending on the shop's 'ability' to overcharge. If you do it yourself then it the bus trip to sim lim and the price of the 9V battery clip and juz solder 2 connections plus heat shrink tubing. Unless you really know nothing about soldering and electronics then no choice...
