I had to find a solution to this too, as I am a chronic flavour changer, & currently have at least 50 cartos in use! Writing straight on the carto with a Sharpie was ok, but it does rub off, so I bought a package of large (ie:standard paper size), clear printable stickers
I measured around the cartomiser adding a little for overlap, & made each about 5mm high, then made up a template of 100s of cells using MS Word.
I already had a list of all my flavours & mg (so I could rate & reorder my favourites) & simply typed them into the template, repeating them to put onto future cartos. Those that I have large bottles of, I just copied more cells of. I also left some blank cells for new flavours, to write on with a sharpie.
After printing, I gave it a light spray with sealer, then used a knife & ruler to score the edges for each without cutting right through the backing.
It sounds time-consuming, but it's now so easy to just grab a preprinted label when I am filling a carto!