Any guitar teacher will tell you to practice with a metronome. Always. Any guitar novice will tell you that worrying about timing while trying to figure out finger placement, transitions, strumming, and every other concern is impossibly difficult.
Keeping consistent time is something that I still struggle with. At first just strumming on the beat at normal 4⁄4 time was a tall order. Nowadays I’m working on accurately hitting my sixteenth notes even across more complex strumming patterns like the one from Ziggy Stardust, but it’s still very much a work in progress that needs regular practice.
My guitar teacher gave me a pretty interesting tip the other week for practicing with a metronome. I can reduce it to roughly this:
Run your metronome on two beats per measure.
So for example, when practicing in 4⁄4 time, it’s tempting to keep your metronome at the same. This works fine, but if you miss the start of a measure or make a mistake, you need to wait until the metronome loops back on itself before you get your next chance to restart. So if you screw up on the first, you need to wait for three more.
This doesn’t make much difference for one or two measures, but given thousands of hours of practice, it adds up.
By comparison, practicing in 2⁄2 is functionally identical, but if you miss a note you can get right back into it on the next beat. It can also save you time reconfiguring your metronome as you move between songs.