The Business.com team recently published this guide to Unlimited PTO – How Your Business Can Offer It as a Company Perk which may help answer your question. Unlimited PTO is a very progressive policy, one that you need to make sure the company can handle before implementing it. Your employees need to value autonomy and be committed to the organization's goal, or the switch could be very challenging.
The best way to introduce unlimited PTO to the company is by having managers set an example. Management needs to be exercising the new policy for the rest of the team to follow. Also, be sure to explain in your announcement why you are switching to unlimited PTO. Is it because you value each employee's quality of work and effort more than the number of hours they spend in the office? Or is it to save money on time tracking software? Or recruit better talent? All of the above are good reasons to change your vacation policies. There is no reason that employees shouldn't be able to take time off if all of their work is getting done.