
Back when i was mainly in the political blog genre, i was "recruited" for this (or specifically, they tried to). I went along with it long enough to find out what the deal was. It required doing a writing test that shows the person's "persuasive writing" ability. At least with the people who offered me work, it went like this:
A "Contractor" who is a third party "cutout", hires people to shill. The individual "free-lance" shills are usually not told who the actual "Client" is (although they can often guess because of the message). The shills are given their next "projects" (sometimes as a team), given info on how to mask their MAC numbers and change I.P.'s, and sent a URL for a forum to go to, and of course the specific "message" they are supposed to disseminate and push there. Sometimes forum names of people there are also given to "support" or '"attack"; and they are often provided background info needed for them to sound "up to speed" on the subjects discussed (lol one way to detect them is they will often use "buzz words" indiscriminately and inappropriately as a means of trying to establish "cover").
Sometimes, existing forum accounts are used by giving the new person a pre-existing account name and password. This gets tricky, as people can often exhibit a strange and innate subconscious ability to detect differences in writing style (whether this is "ESP" or simply "intuition" is hard to say, but it does exist). Those that do this, usually keep their posts as short as possible... The less said the better.
The targets are not always open forums, but often news sites (like newspapers' online sites, or news orgs' online sites like "CNN"), that have popular "Comment" areas under the day's articles.
The intent is not always direct, but sometimes more subtle, in that the threads are often attempted to be "shaped" and "guided" by a team working together in certain directions first for a while as a "set-up" for some specific reason that later comes out. Not only that, but there is frequent use of something called
"REVERSE BIAS"
which means that the shill will often start out new on the site, posting in favor of the thing they are actually against. This can work two ways: "One", as setting a "provenance" and "bona fides" for the shill, and "Two" as a means of subtly dis'ing a subject by pretending to be for it. Often in the first case, the person will later "change sides"... LOL as a means of saying: "Gee, i was convinced by the wonderful arguments on the other side that i was completely wrong before! Thanks for saving me from my own stupidity!"
And YES, i did tell them to got to hell
Lawyers who are out of work or currently "under-employed", are often tapped for this work... Either as free lancers or contracted through their Firm. Lol i guess the idea is that they are better than average at Debate (since this is actually taught in Law School).... But the truth is, very few of these people no mater where they come from, are any good at all When it comes down to it, they usually "suck" and their main strategy is to often use ad hominem personal attacks and threat of derision. In fact this is a main indicator to watch for... Not that many adults will stoop to these tactics unless first provoked. But some will who are not shills lol, so it is only ONE indicator and not "proof".
Shills can be effective on less intelligent people who are confused by more arcane arguments, by re-defining the argument into a more simple context, then once that is done they use straw-man arguments "proving" the new, simpler thrust that meets their goals... Basically it is a "bait and switch" game; and classic misdirection... Tarring all the subject by defining only one part of it.... Lol, sound familiar?
"Shilling" is not a new profession (lol if it can actually be called a profession, like "egg sucking" could ), but it has changed and grown greatly since the rise of the Internet.
I have read that the U.S. military uses actual military personnel to shill directly for them as well in at least some cases. But it would be hard to prove that one i imagine
But i've said this before, and will again prolly, lol:
The majority of people suspected of being "shills" or "dis-info artists" on these sites (which are the same thing imo), are probably not really. They are simply people who disagree with us So caution is suggested, and it is rarely wise to directly confront them (unless some good proofs exist first.. Which ARE possible to obtain).
Having said that, they certainly do exist and there are probably some reading this thread right now

The fun part of it is, they are just "people", and are nothing to fear as they are just jerks without any personal honor sitting at their computers... And they certainly can be manipulated, the same way they try to do with others