I’m not remotely proficient in music engraving, but I found it pretty straight forward to type a few pages of music into Lilypond format a few years ago, just using a regular plain text editor.
Comparable to writing (La)TeX for mathematics I’d say.
I don't have any samples handy, but here are some pointers:
1. Before starting, number the measures on the original score. I write the number every fifth measure.
2. Before engraving any actual notes, I'll make a voice that is entirely "skips", plus time signatures, key changes, barlines, etc. I include this voice on each stave.
3. I always try to deconstruct the piece into individual voices. Of course, this is straightforward for many pieces (choral and instrumental especially), but every piece with good voice-leading can be broken up like this. My music theory background (I was a Music & Comp-Sci double-major in college), along with my experience as a chorister and organist, helps me a lot with this.
4. Engrave one voice at a time. For some pieces, this means that I'm engraving half of a stave's notes (or less) in each pass.
5. In most cases, I put a single measure on each line. I make sure to use bar-checks whenever possible (once in a while they won't work, if I'm doing something weird that confuses LilyPond). Every fifth measure, I add a comment with the measure number (to match up with the numbers I've written on the physical score).
6. I use LilyPond's various built-in checks (bar number checks, octave checks...) liberally. They're very similar to unit tests, in that they allow for much easier "refactoring" down the line.
7. The right environment helps a lot. When I was doing tons of engraving, I had a document holder with a movable marker to help me keep my place. I rotated one of my monitors to portrait to show a full page; I'd keep the markup on the other monitor, and had a keyboard shortcut set up to easily run LilyPond as needed. I'd often run it after making just a change or too.
8. LilyPond's layout engine is astonishingly good, but I'd still have to make tweaks to the final output in some cases. I had a macro set up so that I could do a quick /moveGrob to tweak the extra-offset property of a grob to move it without changing the behavior of the layout engine.