I am a composer who started on computer software to write, but am now switching to handwriting music. I have read that one writes everything out in pencil first, then goes over it in pen, but how far does the pencil go? Do you write all lines, all key signatures, all time signatures, and all bar lines in pencil first in addition to notes and rests, or do you do the former list of items in pen first? I'm just worried that if you wrote everything out in pencil, absolutely everything, upon erasing, you would destroy the paper and remove some of the staff lines and things such as that.
Composers of the past few centuries have settled on a compromise: they tended to use staff paper with five-line staves already written in, like so:
They then wrote in everything else with pencil: key signatures, time signatures, accidentals, pitches, etc. This way, any erasing they did preserved the staff lines. Although, in my experience with manuscript study, they didn't erase anything, they just scratched stuff out like a banshee.
And more practically, composers rarely felt the need to write the key signature on every single musical staff; once they write it once, they just assumed it was in play until they changed it. Notational requirements like that can wait for the proper engraving process.
This is a bit of an addition to Richard's answer:
I do compose semi-regularly, and when I do that, I often handwrite the music first. So you may find my procedure useful.
I write into music notebooks (= notebooks with pre-printed staves, a bit like what Richard shows). These should be easy to buy, as well as individual sheets of paper, if you don't like notebooks that much.
I'd just make sure that:
There are not only the staves themselves, but thin guidelines all around, both horizontal (that make it easy to make all ledger lines on the same level) and vertical (dtto for barlines). It makes the writing much easier.
It has a decent paper size. (I use A4, many of the staff papers are even larger).
If it's a notebook, it has a "normal" binding, not a "ring" one. (The hand can hurt quite a bit from writing from over the rings.)
When equipped with such a paper, I just take a pen and write, and when it's written, I go to the computer and typeset it in Lilypond. When I make a mistake, I just cross it out, or make an arrow to a notehead with label "this is a D" or whatever like that, and continue — so no pencil is actually needed. (I also use a lot of "shorthands", like arrows that "copy & paste" bars into different places. Nobody will ever read that — the main product is the Lilyponded score.)
Here's a little sample from my notebook: If you look closely, you will notice that there are very thin grey lines all around the place. Those are the "guidelines" I keep talking about. You can also see some things scratched out and a note "ještě 3×" = "3 times more", which would be of course unacceptable in a normal score.
You can prepare the sheet with the staffs, names of instruments ore voices, clefs, key signs (and even the measure lines), then you make a copy of the empty original sheet (grand staff for piano or full score for band or orchestra). I did so when I wrote for brass bands or a quartet by hand. I never started with a pen, there were too many corrections. Only the final version was a new copy with pen. Some scores remained in pencil writing.
I'd suggest using manuscript paper marked with pre-printed musical staves. Or you can buy a five-nibbed pen for drawing your own staves. If you're reasonably confident about the bar lengths then you can prepare the page by drawing in the vertical barlines in black pen. That way you're less likely to run out of space at the right-hand end of the stave.
However, I should say the creation of the score as you compose and the subsequent preparation of publishable score and parts afterwards are two different processes.
I'd suggest choosing processes that let you maximise the time you spend composing.
I've created scores and parts by hand in the past, and I wouldn't willingly go back. Now for instance, if I want to add extra bars or an entirely new part to a score, I can do it and have beautiful printed parts in minutes. Making the same change for handwritten score and parts would require multiple pages of the score and all the parts to be rewritten - hours and hours of work. It's still creative work, but graphically creative rather than compositionally creative.
In addition, if you're creating a score for transposing instruments, the manual preparation of parts from score can be a lengthy, error-prone process, particularly if you're dealing with unfamiliar instruments.
But having said that, a well prepared hand-written score is a thing of wonder.
If you're the sort of person who works it all out in your head before you write it down, then you'll have no need for any pencil. But look, in this day and age you can choose pen or pencil based on what's comfortable for you. You can always use a copy machine to turn pencil into something indelible that will look like pen.
If you use pencil, sharpen it with a knife and give the lead the shape of a calligraphy pen. This will make it easier to get the fat stuff with one stroke. (You can sharpen a handful of pencils and then you won't have to stop to sharpen as often.)