Reddit mentions of Studying Rhythm (3rd Edition)

Find a used version of Studying Rhythm by Anne Carothers Hall. It's obscenely overpriced because it's a university text, but it is SO good. It's what I used as an undergrad.

The beauty of this book is the progression of musical exercises that have more than one part, intended to be sung/spoken, tapped & clapped simultaneously. It starts with simple quarter and eighth note passages and ends with complex rhythms, changing meters, and polyrhythms.

Everyone talked about practicing with a metronome but didn't explain HOW to do so.

I would first spend some time with just the metronome itself, practicing clapping different subdivisions at different tempos. Clap four bars each of whole notes, half notes, quarter notes, eighth notes, and 16th notes. Count the subdivisions out loud as you do so. Then add quarter note triplets and eighth note triplets. Then practice different rhythmic cells made up of combinations of those subdivisions, mixing up your patterns randomly. When you can comfortably switch between subdivisions or patterns without getting ahead of or behind the beat, you can try the same exercise using guitar scales or singing simple patterns, again mixing up subdivisions and patterns at different tempos. But only move on to something more difficult when you are 100% sure you've Got It, physically and mentally. This might mean just clapping quarter notes and eighth notes for months.

Once you're used to hearing and playing those small skills along with a metronome, you can introduce it to your more regular practice routine. Work out small sections of guitar strumming patterns with the metronome, and then work out small sections of lyrics, paying attention to how they match up and relate to the metronome. Take note of what beat subdivision all of your vocal phrases begin on, and whether it's before, on, or after beat 1.

You want your theoretical understanding of music and physical technique to match up with the complexity of what you're trying to create, so you have the framework to determine if you're succeeding or not. You should be able to know, even if you need to think about it for a second, what beat a phrase is supposed to start on etc.

Rhythm is a skill in it's own, if you can read the notes fine but just not play in time, I would suggest practicing just rhythm exercises via clapping/tapping initially. Tuning your internal clock to learn how certain time signatures and how various rhythmic patterns feel takes practice. I was in your same spot, but I took great care and time to correct it. I would first look at your piece and clap/tap the rhythm while speaking it out loud (i.e. 1 , 2 and, 3 e - and - ah, 4). Try to make a habit of keeping a constant pulse (try tapping your foot to what ever is keeping the beat). Once you clap it out, add solfege (don't worry about pitch, your just learning the notes). Now, pick up the guitar and speak the rhythm/solfege out loud while playing it. Take your time, practice 2-3 measures only at a time. Once you have those, add two more. Once you get your ear and rhythmic phrasing down, you can start to isolate phrases and practice those. In time, you will learn patterns, shapes and have a library of common structures to pull from.

Edit: In school we use Hall's Studying Rhythm. You may think it's expensive, but this book is really invaluable and a great tool.