It is very simple and safe, click inf the following link: ( Official site of Costa Rica National Parks Administration)
where you must fill out the registration form with basic information. Once the information is completed, the system tells you that you can buy the tickets, then only choose the day & the time (For Póas Volcano) you have to put an identification number or passport and voila, the system tells you the total Charge ( $15 for Tourist & $ 2 for locals & residents ) and you make the payment with a credit or debit card. You will receive a confirmation in your mail, just show up 20 minutes before the reserved time.
Once you arrive at the park reception the staff will guive you a small safety talk and then walk for 10 minutes to the crater of the volcano. Normally the time allowed in the crater is 20 minutes. Somethimes and according to the activity of the volcano you can walk the trails or visit the lagoon.