When I build a kit design, but not from a kit (ie from old scanned plans off t'internet) then I normally print out the paterns on paper, then use 3M "Spray Mount" to attach the paper to the balsa then cut through both and peel the paper off later on.
But I ran out of Spray Mount, so I tried a method that I read about. This means printing the parts out onto paper with a laser Printer/photocopier (NOT inkjet) placing the paper over the wood and then using Mrs Phugoids Iron to melt the toner to the balsa, and guess what? it works a treat! It does bow the balsa a little but I simply ran the Iron over the back and it went flat again.
Just thought I'd pass it on as it's simple and as cheap as chips.......
Andrew