I mentioned this the other day regarding obsessive DP thoughts so looked for a page that explains what it is and how it's done so it might help people. The best times to do this are when you are dozing or half-asleep. That's when your critical mind is at its quietest and the link to your subconscious the strongest.
Rather than stating something like, "I am healthy and happy," which might not be true, I find it better to ask, "Imagine I am healthy and happy, how would that feel?" which is only a suggestion. It doesn't involve force or willpower, but gentle prompting. Don't try to visualize, just build on a general sense of how you wish to be as though it has already happened.