I would agree with much of what's been said. There are lot's of highly skilled joiners in Shetland, so a high skill level is important. Secondly, quality of work - that means recognising that for exterior work at least, work must be completed which will withstand Shetland's weather extremes rather than seeking to do it cheaply. Well finished work that's secure and long lasting will quickly gain you a good reputation which in turn will lead to more work. You'll also do well if you're willing to carry out small jobs which many of the firm's here aren't currently interested in.