The links on this page will bring you to a variety of other webpages.
Unlike the Touro databases, which offer a fairly consistent searching experience, each of these links is organized by different companies/organizations/universities/individuals, who are approaching the task of making sources available with a variety of resources, interests and perspectives. Therefore--your experience using each of these might vary a great deal from link to link.
With that in mind, how do you best approach your task?
MOST (but not quite all) of these sources will allow you to sort or filter by date (or at least have their primary sources organized by date). Know the era or year that your historical event occurred, and find sources created around that time.
For example, if I'm researching the Black Plague, and I look on the Avalon Project website, I will choose the Medieval (400-1399) section, and then scroll through the primary source list until I find a source from the 1300s.
SOME (but not most) may also organize their sources by theme. In that case, you may have success by looking for themes that relate to your topic, or your perspective on your topic (i.e. gender, economics, religion, etc)
Keep in mind that most primary source "titles" may not include all the information that might be useful for you to know. Many primary sources are titled something like "Letter from [individual]," or "Charter of [name]". That's great if you're looking for sources pertaining to that exact individual or place, but not very informative otherwise!
For example, the vaguely-titled "Ordinance of Laborers" in fact relates directly to the economic impacts of the black plague.