I'm not good at a lot of things, but I'm actually pretty good at this!
If you're playing something, particularly something that's a little repetitive (as most songs are, especially if you're not singing over them), you're just adding some variation or accentuation to a section. Because I don't sing when I play, if I'm playing something familiar, I might try to alter the chord to emulate the sung part. Not fully, just to accentuate.
I usually start by getting the song down. Once I've got the chord changes and rhythm down to my satisfaction, then I'll think about what seems too repetitive, or where a sung part (or another instrument that's not present) and try to inject something. Either lift a finger, or drop an extra finger, or a slight shift somewhere. Play with it. If it sounds right, try it again.
Now, this part is optional, but if you want to KNOW what the chord you just "invented" is, you can usually find it here:
What's the name of that ukulele chord? Just select the notes of the chord on the ukulele fretboard and we'll find matching chord and key patterns.
ukebuddy.com
I've run into a couple that were weird and didn't show up there. Strange extensions like a A7 no5 #9. Almost anything can be "defined" as a chord if you think about it long enough! The only question is, does it fit your song, and can you reliably reach it? So, you can either try to look up possible chord variants beforehand... or you can just experiment and see what works.
Example:
Ukulele and Guitar chords with free download
ozbcoz.com
Simple 12-bar blues. Pretty much sounds good no matter how you play it. But, you can add some cool variance to it by doing stuff like this:
The second line is kind of long on that C7. Just drop a finger on the 3rd fret and turn it into a C in the middle of that line!
You can also use the same finger on the 3rd fret C note to turn a G7 into a G7sus4. It sounds all fancy, but it's really super-simple!
I don't think it works well for THIS song, but you could also take that D7 and LIFT your finger off of the 3rd fret A-string, and if you barre the rest of that chord as I do... you've turned it into a D6.
Once you start developing a mental library of these variations that work, you can insert them wherever you like and see how they work!
Basic G chord, lift off of the A string = Gsus2
E7, lift off of the G string = Em7
E7, lift off of the A string = Amaj7sus4
E7, drop your pinky on the A string 3rd fret = E7+5 (I don't even know what that one means! But, it sounds cool sometimes!)
One-finger C chord, drop a finger on the E string 3rd fret = C5
Same C chord, drop a finger on the E string 2st fret = Csus4
Alternate between C and C7 (or Cmaj7) by just moving one finger along the A string
The possibilities are not endless, but they are plentiful! Pick any chord that you know, and play "what if" games. Do that, and play with some strumming variations, and you can almost sound like one of those "good players".