Lead was banned from residential paints in 1978, but most repurpose-worthy wood received its first coats well before then. It doesn't pay to guess whether the paint has lead, and the chemical lead-test kits available in hardware stores are unreliable, so simply assume the paint has lead and treat it accordingly.
Lead paint doesn't become hazardous until it dislodges from the wood in the form of dust or flakes and becomes ingested. So if the paint coating appears solid, simply recoat it with more paint or a clear sealer such as polyurethane. To prep the old paint for its new topcoat, wash the surface with a cleaning agent and rinse off any residue with a water-moistened rag.
Wear goggles to keep paint chips out of your eyes, and don a respirator with cartridges approved for toxic dust. It should have a pliable rubberlike material that forms an airtight seal around your mouth and nose. Commonly available cloth/paper dust masks do little to shield your lungs from harmful particles. Wear sturdy neoprene gloves as well as a long-sleeve shirt, long pants, and shoes that can be washed. Or, wear disposable coveralls and boot covers.
Despite taking these precautions, there's still a chance you will loosen some lead paint that goes uncollected or airborne. You're better off if that happens outside your house rather than inside it. Put down a plastic sheet to catch debris.
If the old paint has deteriorated to the point of flaking off, you need to remove the loose material with a hand scraper. Otherwise, the loose paint will continue to fall off. Find a flat scraper that's comfortable to hold and be prepared for a little work. You can even buy curved "profile" scrapers for getting into moldings and tight spots. Avoid sanding by hand or machine; sanding generates clouds of dust particles even if your power sander has a collection bag or is hooked to a vacuum.
To control the dust and flakes generated by scraping, keep the surface moistened with water from a spray bottle as you work. Allow the wood to dry thoroughly before proceeding to the final step.
Fold up the plastic sheet and toss it out, along with the gloves and coveralls/boot covers if you wore them. Use a high-efficiency particulate air (HEPA) vacuum if you have one. Don't use a common shop vacuum; it will only spread around any fine dust it sucks up. Wash your clothes and shoes in their own load, then shower off.
Apply a clear, film-forming finish or paint to seal in any remaining lead-based paint.