Stevies right, to the extent it's actually worth buying a 'big' iron to make this job easy. 40W (or more) is about right.Try to use an iron large/hot enough for the job
Drill a hole in a piece of wood to hold the banana. First heat up the wire and cover it with lead. Then heat up the banana sticking in that piece of wood, and add a little lead to it as well. Then whilst still heating up the banana, stick in the wire and wait a second till it all connects. Voila.