The details of privateering are well covered in David Starkey's "British Privateering Enterprise in the Eighteenth Century" - published a little while ago now, but still the best book on the subject.
He shows that most it was usually existing shipowners who fitted out privateers - "...shipowners could seek profits in trades peculiar to periods of hostilities". This could be either to lease vessels to the government to serve as transports - sending them as a 'runner' (out of convoy - faster but riskier) - or procure letters of marque for their ships which would mean they were "...acquiring for themselves, together with the ships crew, the sole interest in any enemy property that might be seized".
He states that many owners did so, either by setting out a ship specifically for the purpose of capturing enemy traders or simply continuing with commercial voyages, but possessing a LoM, which gave them the potential for captures.
Nevertheless, it was a distinctly risky business. They were not paid for this - the only money in it was by making a good capture - and there was no guarantee of making any captures, and if they did, it was then liable for a legal process of condemnation that could sometimes take years in some cases.
There were different types of privateer/LoM - there were a few 'deep-water' privateers, organised by a company or group of merchants to seek out enemy trading vesssels by cruising in likely waters, but many more 'armed trading vessels' - ships which continued to carry on with commerical business, but could take the opportunity to engage in commerce raiding. Many of the privateers fitted out in the Channel Islands were small craft, evidently aimed for use against French coastal traders.
There were a small number of organised expeditions by a group of privateers - Woodes Rogers is an example - but they seem to have preferred keeping to themselves - no sharing of potential profits.
Training? there was no organised training for RN seamen, let alone privateers - I would think it would be described these days as 'on the job training' - i.e. you learned by experience.
Incidentally, the Letters of Marque are being transcribed by the National Archives, with details of owners and officers - this is an example :
http://www.nationalarchives.gov.uk/cata ... ssmethod=5