Easy enough,
Semi-randomly; when I sit down to do some editing, my starting point is always one of 3 places: Monitoring new page creation (New Page Patrol), monitoring recent changes or existing items on my watch list (items accumulated from the above 2). Any article I tag for deletion was either originated from #1 or #2 above, or was linked to in one of the above.
Generally I don't discriminate; if I see 3 very similar articles worthy of deletion or at least a deletion discussion (AfD), all 3 will be up for a nomination. I qualify that with "generally" because I don't always have the time/energy/desire/etc to do all that work. In that case I'll add the others to a "to do" list or my watch list. However, if 1 or 2 of those 3 articles are better off (e.g. topics that are clearly notable, but no one's taken the time to dig up sources, or the article already has better sources), I'll use the appropriate cleanup tag (ones that indicate the article needs additional sources). It always comes down to the quality of the sources; a single mention in a nationally published news source (e.g. NY Times) carries a lot more weight than scattered blog postings or trivial mentions.
Cheers,  (talk)