Visas for other countries
You would normally need an invitation letter from the country you intend to visit. For a tourist visa, it would be better to get one from a relative or friend rather than a Priest, so that the consulate would not think that you might go there to work. The invitation could be addressed to you via an email containing the full name, address and cellphone number of the one inviting, as well as the dates you are being invited to go. There is no official format for such invitation letter, which should be kept as simple as possible.
For online applications: you may normally still change your appointment date. This is because other people might suddenly have a last minute change of schedules, leaving their original slot open. You might be lucky and get a much earlier date.
For the Rector’s certification for your Visa application, download and follow the instructions from:
(Just adapt the details for other countries.)