Cover the flooring adhesive with old, plush towels.
Pour boiling water liberally over the towels while wearing closed-toed shoes and long pants. Keep the soaking towels on the subfloor for an hour to soften the adhesive.
Scrape the softened adhesive with a floor scraper, removing as much sticky buildup as possible. Always scrape hardwood subfloors along the wood grain. Do not scrape the subfloor with so much force that gouges form.
Vacuum the subfloor thoroughly with a wet/dry vacuum. Extract all loose adhesive remnants from the subfloor.
Inspect the subfloor for stuck-on adhesive residue. If any sticky residue persists, heat the stubborn adhesive with a hairdryer until the residue softens. The older the adhesive, the longer it takes to soften.
Scrape the softened adhesive remnants completely off the subfloor. Work quickly to prevent the adhesive from hardening again.
Vacuum the subfloor thoroughly with the wet/dry vacuum. Remove every last loose adhesive fragment from the subfloor.