Take your time! Other than that, like speedprotege said. Fit the UL's, get on the bike & make sure they're in the position for you. Measure the locating holes in from the ends on the stock bars. Scribe a small cross, take the bars off, centre punch cross, drill 5.5mm hole. Fit bars, go & enjoy!
I just installed UL's and I concur with everything above.
To mark the hole locations, I put a bit of Sharpie marker near the place I wanted the hole to be, and then put the switch exactly where I wanted it, clamped it down just a bit, and rotated the switch back and forth just a little. This put a mark in the Sharpie ("permanent" marker) that I could easily center-punch and then drill. The marker didn't get into the bar finish at all - I wiped it away with alcohol.
like everyone stated.... before you drill.. get everything lined up how you want. put the grips and levers on the bars etc etc.
Thats how i did mine.
To mark the holes i ended up using some spray paint into a plastic cup, then used a q tip to paint the lil indexing pins in the control pods, then carefully clamped them down on the bars where i wanted .