One thing that I have learnt is that there's always more than one ways of performing something in Spoon, therefore there's not a BEST step!
If it's a simple substring, then use String Cut.
Maybe you can even look at the String Operation step.
Another option would be Regexp or even "Modified Java Script Value" if you know some java.
Actually, you could even use "Split fields" step if you use delimiters within a field.
I was starting to suspect that "there's always more than one ways of performing something in Spoon" :-)
Many thanks for the last suggestions!