Here's one way to do it in Houston (Harris County):
Step 1. Drive neighborhoods with vacant houses (Houston has a lot of these) & record the addresses. Try to get 75-100.
Step 2. Research the taxes owed at hcad.org (look up by address and clip on the 'taxes' link.
Step 3. For properties with 3yrs+ delinquent taxes make a list of the owner's current address & the amount they owe in taxes.
Step 4. Try to get their number and call or just mail them a letter (as an area developer) telling them the house is in bad condition, it has delinquent taxes, and can be eventually taken and sold at auction if THEY don't do something. Tell them you will pay their taxes, their liens, & give them "several thousand dollars spending cash", & buy the house.
*Pull low comps in the neighborhood to justify a low offer to the owner. When you get ready to sell, you'll of course pull high comps.
Last, if they're interested, negotiate and get it under contract. Immediately send the contract to your title company for a title search. Add up the taxes, the liens, and what you're paying the seller and that you're real purchase price, (X).
If Flipping: Find an investor who wants to rehab it and flip it to him for X profit. (have 5-10 investors ready always) Sign another contract with your investor for (X + your profit) and send that contract to the title company also. Tell them you're doing a "double closing". Close with the original owner at 10am, and close with your investor at 10:30am the same day. You keep the difference in contract prices.
If Keeping: Fix it up, refi cash-out, and section 8 it.
6 or 7 or 8 of these per year can make you $100K. Of course, there are factors and variables specific to each deal.