many ways do do it now. I use the dremmel, but not all the way through the metal of the carto. Just dremmel down until the metal looks thin, or it doesnt really have to be that thin. Wipe off any shavings, then punch with a hammer/nail. This way, the area that you shaved off will hold the nail in place better and no shavings go inside the filler because you didn't dremmel all the way through. I also use an incense holder to steady the carto. Be very careful when you punch your hole, not to go too far into the filler or else you will end up with gurgling and/or a slow leak. The nail you use is important too, I don't know the proper terminology, but basically it should go from a point and then as close to 45 degree angle you can find to the nail post/rod(?). Like this (good nail to use on the left/ the nail on the right will punch too small of a hole or go too far into the carto):
I know my nail is rusty and gross, but hopefully you can see the wide angle.
Hold a carto up to the bottom cap of your tank, equivalent to where it would go in the tank and mark with a sharpie 1/8 to 1/4 of an inch above the top of the end cap. Dremmel and/ or punch there. For thin juice just tap the nail through lightly, for thicker juice tap the hole larger 2 or 3 times (hold just above the tip of the nail with your thumb and index finger). The consensus of a good size punch seems to be about 3/32 of an inch. I learned by trial and error and basically eyeball it. Most juice is good with two punches this size on opposite sides. Again, try not to go too deep. Once I've made my initial punch, I will even turn the carto slightly and make the punch larger at an angle to avoid puncturing the center.
mark slightly above the bottom cap of your tank...
dremmel notches where you marked to prevent the nail from slipping around and make punching easier(optional)...
hold above the tip of the nail, use something to keep the carto from rolling, tap into the spot you notched...
after lightly tapping a few more times you will have a punch like this on both sides, good for thick juice. Use a smaller punch for thinner juices. Sorry about the cheesy pics and good luck. There is a jig and punch set available somewhere( ask around) and also a new thread where someone had some luck with wire cutters( I'll look for that link)
ETA on this: those punches pictured will really only be good for very thick juice. Most juices you will want to keep it a bit smaller. With regards to the wire cutters, there is a new find people are raving about: 1/4" brass self-tapping saddle valve at loewe's. I just picked one up that says 3/8", part number LFA-141. I havent tried it yet, but people are saying it works perfectly. I know its off topic, but just wanted to share.
I blew my first atty last night trying to drip on my 5v.. 
I'm pretty sure that it was my fault though. 
