You are on the right track by practicing sight reading guitar with a hymnal in front of you. The treble clef in church music is usually a simple melody, and it may be a familiar tune you already know by ear.
So it's good sight reading training. The hymnal, however, is probably written for a piano, so you'll see notes that seem to be "on top" of the same string. See if you can play the top note on an open string, and the harmony note paired with it on the lower string down.
Usually these "twin" harmony notes can be found on adjacent strings on the guitar, a few frets apart.
The problem here is that a note written for piano means only one key, "black or white," on that keyboard, but may be played in several locations on different strings up and down the guitar keyboard.
