What you are asking is like asking how long is a piece of string. It would be impossible to condense aircraft design down into a few paragraphs. If you want to scratch build a plane, buy or download a set of plans for a plane that you know flies well. There are plenty of links to free plans sites in the sticky at the top of this page. Have a look there, and do some research on designs that you like.
'Fly well' is a very open and loose term. But if its a trainer that you're looking to build then that narrows it down a bit.
As I said, look up the free plans and see whats going. Having a .DWG and/or .DXF viewer is a great help when looking at free plan downloads as many are in CAD format.