I'll admit that I'm still a little new to vaping, but I'm a quick learner
I've got a KR8 system and have been using the same cartos for the past month. I just found the PTB method a few days ago and adapted it to some of my more "worn-out" cartos - AMAZINGLY AWESOME, BTW!!!
First off, what I'm finding is happening to the them to "wear-out" is that the polyfill around the wick in the ATTY gets burned and loses contact with the ATTY/wick, resulting in very poor wicking from the filler - along with the persistent burnt taste.
Figured I'd give a step-by-step (will probably work with any similar carto)
I DO NOT RECOMMEND USING WITH AN AUTO-BATT until you do your own testing and find the right amount to use
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1. Remove endcap plug
2. Use suitable tool to GENTLY remove filler material. I use a stripped twist-tie with a crook at the end and slip it down betwen the casing and filler, twist it a little to grab the filler and gently pull it out. I leave the center tube in, but I'm not sure that's even necessary with how the PTB installs...
3. Inspect and clean carto of as much remaining filler as possible and take this time to insure you have a horizontal ATTY and that it's centered over the hole. I found the vertical type don't work very well in general anyway and just toss them.
4. Take a PTB, remove the string, empty the tea and pull all the seams apart to get a nice 2x4 inch sheet (approx). Holes or small tears don't seem to matter, but you can use scissors to cut the seams off if you desire.
5. Fold lengthwise (to make it 1x4) and roll it into a tube as tight as possible.
-At this point, there's several different ways of installing it-
I've been putting it so the folded end is away from the ATTY. As-is, it's going to be a little too long if you're using a rubber endcap. So...
6a. cut or fray the non-folded end and insert toward ATTY
-or-
6b. trim a couple mm's off the non-folded end and insert
-or-
6c. leave as-is, insert and mash it down with a rubber cap (I don't recommend this) or use a hard cap if you have them.
--Side not on insertion--
The inside from a cheap ballpoint clicker-pen can be used as a roller-upper for the PTB. And then you can use the ballpoint tip as a guide to prevent the frayed ends from getting caught on the center tube of the carto. And try let it unroll a little to get more space between the layers, if possible.
7. Fill 'er up!
Normal fill methods all seem to work well. I'm only using approx 15-20 drops since I'm still using an auto batt and still in the experimentation stage. But so far, leakage doesn't seem to be an issue at all.
This material doesn't seem to "fill" the carto as much as the poly, but it holds the liquid in place very well. I'm almost thinking one PTB isn't quite enough, but it seems to be working well so far. Some will claim it's resistant to burning - this may be true to a point, but it WILL melt if overheated! I found that out when I tried it with a vertical ATTY carto - I didn't get a bad burning taste, it just stopped working well and removing the material showed a hole melted where it was near the atty.
I'm going to try one without the center tube and report back

I'll try to upload some pics if anyone's interested.
EDIT: One other note - Because it lets the liquid flow so well, I've found that the liquid may leak a little if the carto is dropped or severely jostled. So make sure to use the condom-caps if you're traveling with them
EDIT2: Just tried one w/o the center tube (I shoulda took pics, d'oh!) - seems to work just fine as long as you don't flood it, lol! I put in 23 drops and that was TOO much! Start with 10-15. Then do the "blow-out" method to clear the hole. It doesn't sound like very much fluid, but it's the next best thing to dripping w/o the hassle of refilling so often, IMO. Oh, and 10-15 seems to alleviate any leaking issues, but I still
DO NOT RECOMMEND USING WITH AN AUTO-BATT until you find the "sweet spot."
EDIT3: Leave the tube in...