If you see a home that you like, you should apply for it! If the landlord thinks you are an interesting candidate of becoming a tenant, a notification will be sent to you. Once this has been done, you can send a message to the landlord and request to go on a viewing.
In other words, to arrange a viewing you and the landlord needs to agree on a date and time.
Make sure to ask as many questions as possible before the viewing to save time. This will hopefully make sure that you don't go on viewings that doesn't accommodate your needs.