Pattern: Made my own based on one of Riley's tops.
Fabric: I sized down an old short sleeved tee that was too big for Riley and the fabric for the sleeves came from an old top of Phil's.
Sewing: It was the first time that I'd tried anything like this and I'm really happy with how it turned out. I used one of Riley's long sleeved tops as a pattern and added contrasting sleeves keeping the existing neckline and hem at the bottom for a quick and easy transformation.
I followed this tutorial here for downsizing my tee and reconstructing it.