If you've already installed Murph's fuse block, it's a piece of cake (just wire it to the new fuse block). I'm sure others will comment as to where to splice into the existing system.
I have a length of appropriate gauge SJOO insulated wire running from the fuse block/J Box area to the handlebars, where it is wired into the switch, which I zip tied against the right vertical handlebar (I did the same with my driving lights, and attached the rocker switch to the left handlebar). I bought some shrink tubing which I wrapped around the switch (to completely cover the wires I soldered to the switch). A fellow rider commented that it actually looked 'factory' since I had done such a good job with the shrink tubing.
The grip heater kit did come with sufficient wiring to reach the fuse block I think, but the SJOO sheath is much more durable.
Others like to locate their switch above the left glove box or other areas, it really comes down to personal preference.
The only 'annoying' thing that has come up with my current grip heaters is that the 'high' circuit isn't working anymore, only the 'low' setting, so I don't get as much warmth on cold days as I'd like. Still, some is better than none.