Reddit reviews The Hymn Fake Book: A Collection of Over 1000 Multi-Denominational Hymns, Melody, Lyrics, Chords

I'm not sure The Real Book is what you're looking for, since you've asked about hymns. TRB might be a bit complicated, with its emphasis on songs that feature extended and more robust harmony. This is the right track, though. A "fake book" of any variety is definitely going to help- perhaps The Hymn Fake Book.

It's sort of a combination of simple note reading, experimentation, and adaption to the instrument. Process:

1. The key is key. Figure out the key. With the sheet music in front of you this is really a matter of reading the staff notation and then figuring out whether your piece is major or minor (C and Am have the same key signature, G and Em have the same key signature, etc.). You can usually figure this out by listening to the last measure of the piece for its flavor, major or minor (usually).
   
   
2. In each measure, read the notes in each voice and write them down. The notes will tell you which chords are likely.
   
   
3. Now a little guesswork. You need to figure out which chord is appropriate to each measure. Usually this involves knowing a bit about chord progressions and phrasing (generally simple in hymns). If you can sing the melody, guess the chords and sing while playing them. Let your ear be the guide; if it sounds right, you are good to go. Also, it is not necessarily the case that only one chord will work with a given measure (if this was a functional relationship, someone would have written a computer program to deduce the chords for each measure).
   
   
4. Chord voicing. You will want to find the best way to play the chord on the mando. This gets into questions of voice leading, maintaining a good bass line, etc. Often your ear will be a good guide here.
   
   Step 3 can involve adapting your chord to your instrument. For hymns, you probably won't have to worry about this a huge amount except for seventh chords, and there are pretty extensive charts available for mando seventh chords.
   
   But really, after all is said and done: This is a pretty easy thing to do, if you know some basic music theory. If not, I think the closest I can get to ELI5 is (a) go read this and this, then (b) go arrange the hymns for mandolin.
   
   I'll try to post an example later this week (arrgh, not on vacation anymore, so much less reddit) for a simple hymn. Or PM me with a scan of a hymn and I'll try to mark it up and show you what I am talking about in steps 1-4 in the context of an example.
   
   EDIT: Another way to practice this chord writing skill: Get a book of hymns with guitar/piano chords already marked, and try out the process I outlined above. This way you can check your answers. This is not a bad start. Heck, it may have everything in it you want already.