Find one, buy it, put it together. Mostly - then stop as I realise the back panels are made of ply, and haven't been painted on the edges - which might be a problem since the bottom edges sit in a cross member that has a 1/4 inch wide groove in it, and will doubtless hold water.
Then realise the inside of the groove hasn't been painted either.
Ever try painting inside a 1/4 inch wide groove, deeper than it is wide, without getting paint on the edges? I ended up using a model makers paint brush :(