works well for me
If you simply want an easy way to hear what you’ve written & to write things down without having to learn all the nuances of manually writing musical notes that are readable, you can download a free musical notation application (I personally use one called MuseScore). It takes a little bit to figure out at first, but that one offers the option of just starting you out with a blank score you can start writing on to get a feel for it.
As for actually getting the ideas in your head onto the sheet music, that takes more time… it may help to use an instrument as a medium — pianos and keyboards are especially easy to translate into sheet music, but other instruments work just as well, really — and get the sounds you want. When you’ve got the sounds, you only need to write them down.
There is a lot that goes into it, but the core skills are being able to read music notation, understanding basic theory about scales, chords, progressions and melody – and learning to really listen to music and understand it in your head (which is called audiation).
While that seems intimidating, you can really start to piece it together and learn step by step. I recommend watching the entire free course, to get an overview of the skills and the way I teach them. The full 101 course is very thorough and will get you writing relatively quickly.