Good first try; flash has given some extra saturation in the colours but the exposure is good; background is nicely underexposed by a pinch and it looks like you've shot with the subject in shade, which is first-rate thinking. Flash direction is good, although I'd have moved it round a few degrees maybe to get a bit more direction on the light to see if a bit more -drama' could have been created. As a first try though. full marks
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TBH, you don't need the Sto-fen.... all it's doing is sucking power from the flash, meaning you have to dial in more than is needed to get the right exposure. It may have a slight diffusing effect, but at the expense of overall power I'd just use it indoors for bounce flash. Get yourself a cheap softbox or a reflective brolly and they'll give much more immediate changes to the quality of the light. I generally use a softbox for my OCF work -
here's an example from another thread - as they're very easy to use to create soft light and drop-off. Plus they're less likely than brollies to suffer wind damage outside.