I installed the grips, then attached the controls.
The controls are a two-piece design so you can take them entirely off the bars without removing the grips. Not all grips are the same length, so I just went with "as close to the grip as I could get without interference with anything." Basically, to cut a long story longer, I eyeballed it.
I get enough traction between the control housing and the handlebar that they don't move. If it really bothers you, you can put grip tape or something around the bars where the controls attach.
If you choose to drill, you can color the pins with something that transfers easily (wet paint, soft pencil, or something) and place the controls where you want. If your paint didn't dry and you didn't rotate the controls around the bars, you should end up with a dot that tells you where to drill.