More than likely even if you do not know the term OSINT, you have used it. If you have dated modern women, it has likely been used against you.
OSINT is open source intelligence. Have you ever “Googled” someone or yourself? Then you have used OSINT. Open source intelligence is using any publicly available information to gather information about a person or organization.
The reason for your search will determine what tools are best for you. Sometimes it is any number of search engines like Google, Dogpile, Bing, Duckduckgo, etc.
When my daughter started dating, she cautioned at least some of the guys that they would be background checked. Now some parents may pay for these name check sites, but my wife and I have learned to use OSINT and do the digging for free. I was able to find stuff that one of the guys did not even know existed (or wouldn’t fess too as he was a crap fest). Once you have gathered your notes form the basic web searches, you can start in with the local civil and criminal courts. I am in Washington State where we have some good searchable resources open to us. Check your area to see what you have access to. Use your own name to see what can be found (without being in law enforcement). You can piece together a fair idea of their character. Are they a felon? perhaps a sex offender, or just a ton of civil cases for not paying bills etc. You could find bankruptcies, evictions, or simple disputes. Some of these results will require a trip to the court house to get more details that are all in the public record. Leaving home may not be technically OSINT as you may have to pay for access to the data.
As you start your adventure into OSINT, I hope that you bookmark the best sites that you find. As you practice, you will go back to those favorite sites and become better with your search patterns. Before you know it, you will be a world class information excavator. (see also, the skills of a suspicious wife/girlfriend)