There are a few DIY wideband circuits and kits around. Google "DIY wideband" for info.
It's not as simple as reading a value. A typical wideband uses a PID loop which controls current through the "pump cell" of the wideband sensor. The amount of current needed to keep this cell at "stoichiometric" indicates how rich/lean you mixture is. You then need to convert this current to an actual Lambda value.
Personally, I'd buy a kit instead of trying to make your own. Especially if you intend to rely on it to tune an engine.