So I have the chords for a song from this website and can play them.

I have the rhythm of the song and count the beat (I always count it as the regular 4 bar beat and adjust my tempo to fit (is this correct?!)).

I just can't seem to come up with a decent strumming pattern. There are covers on Youtube which sound very good but I cannot come up with my own or reproduce them. Any advice?

Is there a formula to work this out?
Time and practice.

If your really new to playing it took me a good few months when I was starting to learn chords. If it something you've never done or done often your fingers will feel awkward. Just keep trying and eventually you'll be able to play your chords clean.