Lols,
I'm sorry if I'm stubborn, but there's no proper way of using burn tool.
For one, if you burn the edges of lineart even just a little bit, it becomes sharp. The contrast becomes too striking.
Besides, with proper leveling, the need to burn is nullified.
Another thing is burn tool is what makes LQ releases sLQ. I don't know with you guys, but I'd much rather see grey dust rather than overleveled and/or overburned sh*t (Censored for the kiddies

).
Learning how to properly use the Lasso tool and the Pen tool EFFICIENTLY, unlike most people here who finds them kind of difficult to use, you can surely IMPROVE your overall Edited Manga skills and Edited Manga quality all at the same time. Using the shortcut (cough*massacre*cough) of the burn tool is NOT the way to go.
Well, IMO at least.
Although, I believe many HQ Manga Editors will agree with me.
Okay, example:
Here's a typical raw image and clean:
Now, case in point Kekkaishi has A LOT of greys. And unlike many Jump mangas, they aren't your typical uniform grey greys... Mostly, they are gradient greys that go softer or harder depending on the shadowing. Fact: that's the Yellow Tanabe's art style.
Another case in point is that many times, magic wanding the blacks will include some grey areas and if you burn them, you mess up all the shadowing effect they were suppose to give off = ruining the art itself.
One more case in point is that LEVELING PROPERLY is ENOUGH for you NOT TO NEED to use the Burn tool at all. You can clearly see from the blown up Raw that there are some greyish specks even in the blacks. FACT: NOT ALL SPECKS ARE DIRT! Sometimes leaving some can act as a good contrast between light areas and dark areas, acting as a buffer in order for the colors to blend together nicely as the cleaned raw shows.
I have to admit that the redraw could use some more touch ups (hehez, find the thing I'm talking about), but it's passable as a good example of the proper way of doing things.
1.) Leveling properly (not too much, not too little)
2.) Blending elements together (aesthetics... lightening up edges so that they don't contrast too much and end up sharp)
3.) Using LAYERS and BRUSH [hardness 100%] ALLOWS for more flexibility and maneuverability rather than using BURN TOOL
4.) Zzzz... want to nap...
Hope this helps...