When it comes to how to whiten dentures, there are several things you can try. Here's a list, starting with the simplest. 1  First step-give them a really good scrub with a stiff nail brush and soap and water.ut if you're short of time, and you want to try to whiten the teeth up for a special occasion, here's a run-down of how to whiten dentures for a last-ditch attempt. Yet again, you MUST have tried all the steps above. Those are the safest and most effective.