Tightening torque: 5Nm, 40Nm... how do I tell?
You do this with a tool called torque (sometimes called dynamometric) wrench.
Without a tool you can estimate it this way:


Make yourself familiar with a weight of 1 kg
Apply the force with your simple wrench 10 cm from the bolt in question


This will give you 1 Nm of force. To get 5 Nm, use 5 kgs of weight or increase length to 50 cm. The math is simple:


  τ = r * F


Where τ is torque, r is radius and F is force (1kg has 10 Newtons of force)

If your bike is expensive racing machine, this tool is a must. By applying too much force you risk making cracks in lightweight materials.