Going to give a quick DIY of how I installed the Curtis hitch on my '04 X. The instructions are pretty straight forward so I'll let the pics do most of the talking. DISCLAIMER: This is how I installed it, however when in doubt, follow all safety precautions and instructions that come with the unit.
Install can be done without all these tools, these are just the ones I ended up having out when I was done, which made the job fairly quick and painless. Total time took about 2hrs and I was taking my sweet time.
Tools used:
Floor Jack and a block of wood
Dremel with cut off wheel
Impact driver
Cordless drill
Tape measure
Sharpie
Tin snips
Torque wrench
6" extension
Face shield or safety glasses
Gloves
10mm socket
3/4" socket
11/16" socket
Bought from eTrailer, everything was complete and arrived with everything needed.
Hardware
First take all the junk out your trunk and pull the spare. Remove the two rubber plugs
Insert the two larger bolts(1/2") and brackets in the plug holes
I put the hitch up and made the nuts finger tight so I can get a feel to see and make sure that the hitch is close to being centered. At this point it's not that critical to get it 100% straight, just wanted to see where center is as the next steps are to trim the bumper.
I ended up using the license plate light as close to center, and made a mark at the bottom of the bumper.
When you make the cut, it should be 5.25" TOTAL across from left to right, and 3" from front to back. Now keep in mind that after you make your center mark, you measure 2 5/8" to the left of center and 2 5/8" to the right of center. That gives you your 5 1/4" width, and keeps you centered. If you put the hitch up like I did to find center, take it down as it will get in the way while you make your cuts. I used tin snips to make the cuts that run from front to back, and my dremel for the cut from left to right. Reason being is I started with the tin snips and they cut fine, but when it came time to make the horizontal cut, the tin snips won't leave a nice square when you're done cutting.
At this point you need to drill 2 holes in the aluminum bumper for the front two holes of the hitch. 
WEAR SAFETY GLASSES OR A FACE SHIELD. Aluminum bits WILL fall directly on your face 
. This part will require you to have to bend the bumper a fair amount to get your drill up into correct position to make straight holes. I would suggest if you are concerned about bumper damage that you remove it to drill these holes. I already have small spider cracks from parking in the city so I wasn't too conserned about anything lol. If you have a fiberglass bumper or lip, forget it you won't be able to make straight holes. Instructions say to hold the hitch up and mark the holes. I go to the gym regularly and it was hard to hold this 35lb hitch up and at the same time us my arm to bend the bumper and reach my hand up to make the marks lol. SOLUTION, hold the hitch up so it fits nicely in the cutout you made, and tighten the rear bolts a decent amount. Then use a floor jack to hold the other end of the hitch up. Presto, you have a free hand.
Now, I marked my holes and as I was going to take the hitch down, I just decided to drill the holes with the hitch up in place. Being that my drill was low on battery power and my corded drill didn't accept a 1/2" drill bit, I used a smaller drill bit in the corded drill, made the two holes, and then used the cordless drill with 1/2" bit to open up the holes.
Now that you have your holes made, you need to fish the front bolts through the bumper support. Curt gives you a cool little fishing line that is threaded on one end. You push the threaded end up into the hole you just drilled and you snake it to the passenger side. NOTE: The line isn't long enough to reach from the hole all the way to the other side. I used some home solid core electrical wire and taped it to the end to make an extension. A clothes hanger may work well too.
To gain access to the wire, remove this panel, 4 10mm bolts
Reach up to the end of the aluminum bumper and you should be able to grab the threaded end.
Take one of the bolts with the bracket through it and thread it on to the end of the wire
Now, here is something that I learned after doing the first one. Don't just pull the wire and shove the bracket/bolt into the bumper. It will hang up and cause you to pull your hair out for 45min as you jiggle the wire and try to shake it free. The aluminum bumper is a rectangle shape, with the longer side running up and down(from the ground to the sky). Take the bolt and bracket and stand it up in the same orientation and then place it in the side of the bumper. It took me 45 min to do one bolt and 30 sec to do the other. When you are pulling the wire, a smooth and constant pull will keep it from getting caught on something and keep the bracket standing up in the same way you placed it. Bolt should drop right in.
Once you are at this point, the finish line is in sight. Put the hitch up and tighten the bolts that are in the bumper first, then do the rear. You want the hitch free as you move it around to get the holes to line up. BE PATIENT AND DON'T RUSH AND ALMOST PUSH THE BOLTS BACK UP INTO THE BUMPER. I almost did this.....twice. I thought about this point of the install when i was marking my holes to initially drill the holes. I wanted to make sure that the drilled holes were spot on, and didn't want to worry about the hitch possibly moving and me not noticing as I was marking the holes. Drilling the holes with the hitch up helped aliviate this worry lol.
Grab your torque wrench and tighten the bolts that are in the spare tire well to 75 ft/lbs and the ones in the bumper to 50 ft/lbs.
Lastly I grabbed some aquarium seal and put some around the brackets that are in the spare tire well
Instructions say that this has a max of 2000 trailer weight and 200 tongue weight. If I remember correctly the G has a max towing of 1000lbs. so use caution when towing! All that's left is to enjoy pulling whatever it is you need to pull!
Link to thread on G35Driver.com
http://g35driver.com/forums/interior-exterior/404715-04-g35x-curt-hitch-install.html