I use the simple cheapie tube cutter that comes with the low-cost dremel assortment pack, it may have even been in the original Dremel 300 box. Pretty sure it's a 426 fiberglass reinforced wheel, but any of the similar ones would be fine. It's definitely not a diamond wheel or any of the other more exotic ones, just a simple fiberglass reinforced cutting wheel.
Cut goes anywhere 1/3" to 1/2" up from the base of the carto (or up from the top of the collar on a fusion). A horizontal cut, ends up being 1/16 - 1/8" high by 1/4 - 1/3"W. Remove the bottom condom from the carto, put the top of the carto, in its condom, into a pair of vice grips lying on a table (or on a paper towel on a table); it doesn't have to be ultra-tight, you should be able to push the carto down into the ends of the vice grips.
Use safety glasses, set dremel to high speed. Make the horizontal cut at that 1/3 - 1/2" height. Don't exert much, if any, pressure pretty much just let the weight of the dremel do the job. With even light pressure it'll only take 20 seconds or so. A slight side-side motion of the dremel head works well, though isn't strictly necessary.
Don't cut all the way through the filler, but a little bit into it is no problem. If you have a little "scab" of carto metal, just pop it out with a thumb tack or small nail tip; I use a 4D nail tip. No need to sand/smooth the slot edges -- I've been doing this over 5 months, the o-rings have had no issue with the slots.
Practice on an old used carto, after even a couple test slots you'll have the hang of it, it truly is an easy thing to do.
For alternate cartos, boge 3 ohm stainless steel are fine, or Ikenvape 510 cartos at 2.5 - 3 ohms. My backup cartos, since I have an 808 fitting, are v4l cool carts at 3.1 ohms, which work quite well at 5 - 5.5V; I have a bunch of them from before the M1A1 came out.
I only use 1 slot, but due to the size of it it is equivalent to multiple punched holes. My eliquid is a pretty thin DIY, zero leak problems (the M1A1 has good seals, key to minimizing leaks).
