I have been asked to produce some musical scores. There is a piece of free software called Musescore which works well enough for music written to a time signature, once you have learned how to use it. But it does not work for chant or for chant-like music written in free time. I spent the best part of a day trying to get it to work. I discovered that there is a lot of discussion on the software's bulletin board but it does nothing more than refer to various work-arounds, none of them easy.
Which means that the simplest option is to copy scanned versions of texts that are out of copyright or to compile sheets from scans that are then pasted into a word processor.
If the music is not available, then there are fonts such as Meinrad which can be used on an ordinary word processor, and if all else fails it can be written out by hand. They also take a bit of getting used to and work in different ways, so it is worth trying various ones.