If you see a home that you are interested in you should first apply for it. If the landlord approves your application as a candidate of becoming a tenant, a notification will be sent to you. It is not legally binding to apply for a place nor to get your application approved by the landlord. Once you have been accepted, 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. This will hopefully make sure that you don't go on viewings that doesn't accommodate your needs.