My son was trying to devise a strumming pattern for a song but gave up.
How would someone go about developing a strumming pattern for song like Chiquitita by ABBA on an acoustic guitar?
In general, I play with a band, so what I play for a rhythm is strongly dictated by the drums. An acoustic guitar in a full band is essentially a tuned snare, so I often try to latch onto one part of the drum's rhythm.
Or, I go to a specific beat and try to play that. I often play a Bo Diddley beat to add a rhythm counter to the straighter rhythm the rest of the band is playing. Or, sometimes, I play on the 2 and 4, like a bluegrass mandolin chop or reggae skank. Depends on what the song is trying to do.
Most guitarists would just feel it, and play. But obviously that's something that comes with experience.
Start by listening to a recording of the song and tapping out a rhythm on a desk with your fingers. In general, you could try to pick out rhythms from a recording's drum part, bass part, and rhythm guitar part. However the opening verses of Chiquitita are against a drumless, fingerpicked guitar part; perhaps go to a later part of the song where the arrangement is fuller.
When you've settled on a rhythm you like, stop the recording, and tap out the rhythm a bit more, to get used to how it sounds. Perhaps sing along.
Once you're comfortable with the tapped rhythm, move to a guitar and try strumming the same rhythm. Especially where there are notes in quick succession, consider making the second note an upstroke. Try strumming the bass strings for downbeats and higher strings for other notes. Experiment until you like how it sounds.
Don't worry too much about originality. Strumming along to a melody is rarely a showcase for invention.
(I note that in the video for Chiquitita, the guitarist is just strumming a downstroke on every beat - but this is only likely to sound good in combination with other instrumental parts)