Generally approaching people at a store or convention hall with "Hey, do you have trades?", "Hey, can I take a look at your binder?", or if you're looking for a specific card "Hey, do you have [specific card] for trade?" should be enough to get started
The kitchen table is a place that loves to abuse graveyards, and you need tools to answer that abuse
Centering is the usual weak point on full-art stock, so only submit copies that sit 55/45 or better on both axes, and wait a few more weeks for the English graded market to establish real comps
Each booster set of 15 cards includes just one white-bordered card, alongside ten commons or uncommons, a test card, and three other rare cards such as a mythic, Future Sight Frame, uncommon land or artifact