Maybe you can play a chord all by itself just fine, but when you strum it, it sounds sloppy. Maybe it sounds like you’re playing rhythmically, but it’s not very good. Maybe you feel like your chords are right, but it still doesn’t sound good. If this is the case, it might not be your left hand that’s the problem. Your right hand might be moving, but it’s not moving with a consistent rhythm or direction. If it’s not rhythmic, it’s not going to hit the strings at a consistent time. Sometimes it will hit them quickly, and sometimes it will be a little slow. This will make the whole chord sound bad.
Try this. Take a chord that is easy for you to make, like an E minor. Strum it, but try not to worry about playing a complicated rhythm. Just try to get your arm to consistently strum down and up. It should feel like a little pendulum. Even if you only strum down at first, your arm is moving consistently. This is more important than getting all the strings every time. A lot of beginners stop their right hand in between strums. They sort of stop and think about it for a second before moving again. This isn’t rhythmic. Try not to stop your hand. Let it just keep moving, even if you don’t get the strings every time. A missed strum is still rhythmic, but a stopped hand isn’t. As you get better, your pick will start hitting more and more of the right strings.
One way to practice this is to play your guitar sort of like a percussion instrument. Take your left hand and lightly place your fingers across the strings. This should prevent them from ringing. Now strum the strings gently, listening for the “tack” sound of the pick. Since you aren’t hearing the notes, you can just focus on your rhythm and motion. Do this for a few minutes, and then take your fingers off the strings and play the chord again. A lot of people find that playing like this helps their right hand get the motion right. It’s easier to hear when you’re not worrying about making the right chord.
You can do this in a relatively short amount of time. Take fifteen minutes. Spend a few of them playing with your strings muted like this. Then spend a few more holding a chord and playing it with the same motion. Finally, spend a few more switching between two chords with a simple consistent rhythm. If you start to sound bad again, go back and mute the strings for a minute or so before trying to switch chords again. With a little practice, your right hand will start to get a feel for the rhythm, and you will start to sound better.